So I understand that when this was mentioned to me in my last session, I understood and took it as try to be flexible with the rules. Though, players really emphasized the Spirit of the Law, and I respect that. However, to what degree and what kind of interpretation do I take that when it comes to going with the rules of a game? 
I understand this antithesis as more applied to legal stuff in the real world, but for 5e gaming... I'm not sure how to translate that. Thoughts?

5e is designed to be loose, with plenty of space for the DM to make changes on the fly. The rules as written are often said to be informal/causally written, with the idea that the intent and flow is more important that hard written detailed rules covering every event. This is quite different to other systems which are more detailed and rigid, which is not good or bad, just different.

So stick to the rules when it's clear and easy core mechanics but feel free to make calls that sound right or make sense in the circumstances. As long as its justified and consistent then everyone should be fine. And as the old saying goes, as long as you are all having fun then you are doing it right. 

RAW = Rules-as-Written. This means, what it literally and actually says in the rulebook, word for word, no more, no less.

RAI = Rules-as-Intended. This means, what the writers intended to say, even though they didn't write that. Many rules are ambiguous, or may be unclear about one thing or another, but the writers had some sort of an intention. The "Sage Advice" stuff is often about this -- "here's what we intended with this rule."

"Letter of the law" in D&D would be RAW - what is written in the rules. "Spirit of the Law" would be RAI - rules as intended, even if it's not exactly or quite what the rulebook says in actual words.

As a DM, you have every right to enforce either of these types of rulings - RAW or RAI. You also have a right to enforce RAW on some things and RAI on others. It is entirely your call, as Dungeon Master.

Now, my view on this is that your use of RAW or RAI or maybe even something else (your own ruling, also called a "house rule") should never be arbitrary or be done to "make it worse" for the players. Rather, your ruling should be based on things like -- does this interpretation make logical sense? (Many rules do not!). Does this interpretation make running my game more difficult? (Many rules do!) Is this interpretation bogging down the game? Or, does this interpretation break the thematic material I am trying to build in my world? If the answer to one or more of those questions is 'yes', then you probably have a good reason to pick one of the interpretations.

By default, as a DM, I use Rules-as Written (RAW). If I am *not* going to use RAW, there is always a good reason, and I will make a written house rule about how we are interpreting it in my game, and what the rationale is for this. I do make some exceptions, wherein I will tell the players "there is a reason, but I can't tell you why yet, because the reason contains spoilers." My players trust me enough to accept that. (Although one, who has known me a long time, suspects that either there isn't really a story reason and I just like doing it this way, and am telling them this in hopes they will forget about not knowing the reason, or else I decided on the ruling first and then retrofitted an in-world reason for it so that I could keep my ruling -- neither of these is actually true, but he probably suspects it.)

I will sometimes do a search for a given rule and see how other DMs rule on it. There are several threads here that I have used as a reference. Other DMs can't and shouldn't tell you how to run your game (and I think you'll find few people here would try). But, if I see that the weight of ruling something comes down a certain way here from other DMs whose logic and reasoning I have learned to respect, then I will lean in that direction as well, just because they have been doing 5e way longer than I have.

This is laid out so much better than what one of my players (who's a DM in their homebrew Starfinder campaign) explained to me.

Thank you so much for putting this in better context as well as translation to dealing with game rules.

First session kinda forced me to adjust a rule that gave players another action to use either as movement, attack, or standard action. That happened, and lead me to later problems with going loose with rules that weren't compatible with the flexibility I made (which was the extra attack feature and the dash action), so I had to flex those too (as in make a whole new game guide with rules coming from the PHB but modded and adjusted to where they're all compatible with each other).

At the end of the say you rules you follow or don’t are less important than the communication you give your players about changes and interpretation. In my experience players don’t like on the spot rulings because they often feel that “if I knew that ahead of time I would of done something else”. It’s not unusual to do a bit of admin at the start of the session along the lines of “last session we did x, in retrospect I would of done it this way and it’s how we will do it next time”

Quote from MavenPShayon >>

First session kinda forced me to adjust a rule that gave players another action to use either as movement, attack, or standard action. That happened, and lead me to later problems with going loose with rules that weren't compatible with the flexibility I made (which was the extra attack feature and the dash action), so I had to flex those too (as in make a whole new game guide with rules coming from the PHB but modded and adjusted to where they're all compatible with each other).

 I would avoid making that kind of change without a lot of thought; 'spirit of rules' should mostly be for situations where the rules are vague or nonsensical, not when they merely prevent the PCs from doing what they want to do.

The spirit of the rules (RAI) often needs context to be interpreted.

For example, you have the Rule of Cool which encourages you to allow anything that sounds cool or adds drama because the intent of the game is to create memorable, awesome stories. But when you take RoC too far, you basically get six year olds playing Laser Wars:

"I shot you with my laser!"

"I dodged it."

"Well this time I shoot you with 100 lasers!"

"I use my Super Laser Blocker Shield that blocks the lasers and also bounces them back at you and you die."

So you need rules that limit unfettered imagination. While the intent of those rules is to provide cool stories, the intent is ALSO to restrict actions to a shared framework that prevents us from just win-buttoning ourselves out of any challenge. Both of these opposing forces determine RAI.

First session kinda forced me to adjust a rule that gave players another action to use either as movement, attack, or standard action. 

I'm not sure exactly what you're talking about, but if you're saying you gave everyone a free Action Surge then that is not even close to justifiable by RAI. The game is intended to have a certain structure, and messing with that structure at such a fundamental level will cause a cascade of failures throughout the system - as it seems you are already beginning to experience as other abilities come to light. You will find more as you go on, I guarantee it.

While their intent may not be malicious, what this really sounds like is that your players are essentially bullying you to change the rules so they can do whatever they want. This does not sound like a game that would be fun to DM. This is your game and you are the boss. I would recommend following rules as written to the letter, only making exceptions when you identify an issue with how the game is working that you don't like. And even then I would do it sparingly. If that goes well, then you could open up houseruling into a more discussion-based format but you need maturity on all sides for that.

Houseruling is a wonderful and powerful tool, but you really need to understand the full implications of the rules you lay down and that only comes with a lot of experience. Hell, I've been DMing for years and years and I still wouldn't mess with action economy at the level you're talking about here. It's like moving your toilet 5 feet to the right - sooner or later you're gonna end up having to move everything else too or you're gonna have a bad time.

That's a better approach I should take if something didn't play out the way I interpreted to be. Thank you :)

The most important rule is the Rule of Fun. I frequently make rulings on the spur of the moment based on how much fun I think my players will have. 

I don’t use that to change the existing rules as written, but the rules as written don’t cover everything and when one of my players tries something that isn’t covered by the existing rules I have them roll a d20 and adjust it as a skill check, saving throw, or ability check depending on which one feels the best and go with the result of their roll. 

At this week’s session they were under water facing small 10’x7’ submarines and one of them cast Polymorph and turned into a killer whale. He said, “I swim down and then swim up as fast as I can between the two boats”. I had him roll and he rolled pretty high so I had the two submarines roll away from him as he pushed them and flipped them over in the water. It was perfect for the situation and everyone loved it. That’s an example of the Rule of Fun. 

Also, don't be afraid to admit a mistake if you make a bad ruling. You ruled one way quickly to keep the session going, but then reviewed it and later realized you'd made a bad call. Just, at the start of the next session say: I said X works this way, but I was wrong. Going forward, its going to work like this instead. Give people fair warning and they'll usually be reasonable about changes. Just don't spring it on them while its happening that you decided to do something a different way.

Quote from MavenPShayon >>

First session kinda forced me to adjust a rule that gave players another action to use either as movement, attack, or standard action. That happened, and lead me to later problems with going loose with rules that weren't compatible with the flexibility I made (which was the extra attack feature and the dash action), so I had to flex those too (as in make a whole new game guide with rules coming from the PHB but modded and adjusted to where they're all compatible with each other).

DO NOT change anything about the combat rules as a new DM. The combat rules as written are there to make the game playable and balanced. They are not perfect. Many veterans will make changes to the rules. This can be done without breaking anything (or breaking much). But as an inexperienced DM, you are not in a position to know what will break the game for you and your players. Leave the rules as-is for combat.

Tell your players that on reflection and consultation with other DMs, you are going to start using the combat rules AS WRITTEN, period. Tell them that as a new DM, you do not feel comfortable with all the modding going on, and it needs to stop. If they express displeasure with this, then ask them who would like to DM instead. Don't be obnoxious about it -- rather be self-effacing. "Well since I am not comfortable running a combat-modded game, and y'all want modded combat, who feels comfortable DMing that way?"

Basic combat allows players to do 3 things, and 3 things only on their turn:

• Take 1 action*
• Move up to their speed*
• Take a bonus action, if one is indicated

Player characters also have one, and only one, reaction, that they can take outside of their normal turn, in response to specific conditions being fulfilled. For example, the Shield spell can be taken as a reaction to being attacked.

* As part of their action OR their movement, but not both, and not as part of a Bonus Action (at least RAW), a PC may interact with one object in an obvious way. Such as moving half of their speed up to a non-locked door, opening it, and then continuing to move through it. Only interactions that don't require special care and aren't already listed in the books as "taking an action" may be used in this way.

Also, there is no such thing as a "Free Action" in 5e D&D. That comes from older editions. (Try it -- on the Combat page in DDB from the PHB, do a word search for "free" - it appears twice, never in conjunction with the word "action.") When people say "free action" they are talking about the object-interaction business I posted above.(*)

Do not allow "another action" on a turn. Do not allow extra movement (except, per RAW, as the Dash action). Do not try to make a whole game guide as a new DM. Follow the rules, as written, until you get your feet wet and are comfortable playing the game as written. THEN you can make your own house rules all you want.

Quote from BioWizard >>

Quote from MavenPShayon >>

First session kinda forced me to adjust a rule that gave players another action to use either as movement, attack, or standard action. That happened, and lead me to later problems with going loose with rules that weren't compatible with the flexibility I made (which was the extra attack feature and the dash action), so I had to flex those too (as in make a whole new game guide with rules coming from the PHB but modded and adjusted to where they're all compatible with each other).

 

DO NOT change anything about the combat rules as a new DM. The combat rules as written are there to make the game playable and balanced. They are not perfect. Many veterans will make changes to the rules. This can be done without breaking anything (or breaking much). But as an inexperienced DM, you are not in a position to know what will break the game for you and your players. Leave the rules as-is for combat.

Tell your players that on reflection and consultation with other DMs, you are going to start using the combat rules AS WRITTEN, period. Tell them that as a new DM, you do not feel comfortable with all the modding going on, and it needs to stop. If they express displeasure with this, then ask them who would like to DM instead. Don't be obnoxious about it -- rather be self-effacing. "Well since I am not comfortable running a combat-modded game, and y'all want modded combat, who feels comfortable DMing that way?"

Basic combat allows players to do 3 things, and 3 things only on their turn:

• Take 1 action*
• Move up to their speed*
• Take a bonus action, if one is indicated

Player characters also have one, and only one, reaction, that they can take outside of their normal turn, in response to specific conditions being fulfilled. For example, the Shield spell can be taken as a reaction to being attacked.

* As part of their action OR their movement, but not both, and not as part of a Bonus Action (at least RAW), a PC may interact with one object in an obvious way. Such as moving half of their speed up to a non-locked door, opening it, and then continuing to move through it. Only interactions that don't require special care and aren't already listed in the books as "taking an action" may be used in this way.

Also, there is no such thing as a "Free Action" in 5e D&D. That comes from older editions. (Try it -- on the Combat page in DDB from the PHB, do a word search for "free" - it appears twice, never in conjunction with the word "action.") When people say "free action" they are talking about the object-interaction business I posted above.(*)

Do not allow "another action" on a turn. Do not allow extra movement (except, per RAW, as the Dash action). Do not try to make a whole game guide as a new DM. Follow the rules, as written, until you get your feet wet and are comfortable playing the game as written. THEN you can make your own house rules all you want.

You can take a reaction on your turn, I think.  You can cast spells like Shield or Absorb Elements on AoO, for example.

You can take a reaction on your turn, I think.  You can cast spells like Shield or Absorb Elements on AoO, for example.

Yes, you can. Reactions happen when a specific condition is triggered, and happen outside of the normal sequence of your regular turn. If you start moving and trigger an AoO, that then triggers a reaction from you, it happens immediately. It is not part of the 3 "segments" (action, move, bonus action) of your regular turn. It is a separate entity.

Quote from BioWizard >>

DO NOT change anything about the combat rules as a new DM. The combat rules as written are there to make the game playable and balanced. They are not perfect. Many veterans will make changes to the rules. This can be done without breaking anything (or breaking much). But as an inexperienced DM, you are not in a position to know what will break the game for you and your players. Leave the rules as-is for combat.

Tell your players that on reflection and consultation with other DMs, you are going to start using the combat rules AS WRITTEN, period. Tell them that as a new DM, you do not feel comfortable with all the modding going on, and it needs to stop. If they express displeasure with this, then ask them who would like to DM instead. Don't be obnoxious about it -- rather be self-effacing. "Well since I am not comfortable running a combat-modded game, and y'all want modded combat, who feels comfortable DMing that way?"

Basic combat allows players to do 3 things, and 3 things only on their turn:

• Take 1 action*
• Move up to their speed*
• Take a bonus action, if one is indicated

Pretty much total agreement here. You can be pretty loose with social or puzzle-like rules but combat has a fairly well balanced rule set. Don't mess with it until you have some sessions done. 

The only thing I would add is that you can move, do something, and move again. But the total movement is your speed (usually 30 feet).

Another think to keep in mind is you don't have to know everything. If a situation comes up, make a judgement call at that time, then figure out the real rule or decide on keeping your ruling later. Let the party know that you are making a call but will figure out the real answer after the game. It is better than stopping the game to look it up.

That is also a good point... the movement can be split. You can move 10', take your action, move 20', etc. In fact a lot of ranged characters use this to hide behind a wall, say. Then move 5' out of cover, take a shot, move 5' back into cover.