So I just wanted to share this. I've played with this group for some months now and today was our 2nd session online, and the DM was too tired. I decided to give it a try since he had already prepared the monsters and maps on roll20. And I had some questions on how to handle certain issues. Party was all lvl 4: some sorcerer, college of whisper bard, path of ancients barbarian.

The first combat went ok, although I found I was really lenient on the possession by 1 ghost when the party of 3 (usually 4 but the DM got sick and i had to step in). How do you guys handle stuff like that, when a party has 0 spells to get rid of almost any status effects. I feel I can't just leave it away because most monsters get really boring if you never have people be in risk of getting a status condition (ie stun, grapple, paralyse, possession, poison, etc). In the end i ruled I had to make the barbarian make a saving throw at the start AND end of his turn seeing how he had a -3 to int. Also it takes a LOT of managing monster HP when a party of 3 PC's goes up against 3 monsters. 

I then threw in a small puzzle, making them use clever mechanics to lure away 2 of the 3 mobs that guarded a key they needed to get to this sessions end boss. 

The end boss of this session was a Mind Flayer Psion. As it was my first time DM'ing i had trouble with 3 things:

1: how to manage HP, because the party hitted for almost 0 except when the bard threw her shatters and when the sorc cast 3x scorching ray on 3 successive turns... You guys got any tips for that?

2: How threatening do I want to be, particularly on a first time DM'ing. Twice I had the Mind Flayer succesfully grapple and stun a target which I then could easily have extracted brain on but I didn't as I find it hard to kill characters people enjoy.

3: What are monsters action economy? Often i found myself wandering (but not doing it) why a Mindflayer Psion couldn't hit 1 person with a spell and then run at another target and try to grapple him.

How would you guys have handled this (as first time DM) and how do you think I did? The party seemed to have fun (Which I know is most important, but I also want to be good at actually DM'ing the monsters and the story).

Your first mistake was running something another DM prepared. Don't do that. You don't know what he or she was thinking when they prepared the session. And there is no guarantee that what the other DM prepped is something you will feel comfortable running. You in fact, saw this happen in your session.

In terms of status effects, you really have to watch it with a small party. In a large party, say of 6, one character going down in the fight to a status effect like paralysis or being stunned leaves 5/6 of the team up, which keeps them at 83% strength. But in a party of 3, if someone goes down to a status effect, they're down to 67% strength. This can mean the death of the entire party. This is why one cannot just pick monsters from the Monster Manual or Volo's Guide based on CR or "experience point" budgets. Those are a general guide, but you have got to look at your party, and judge what that party can do against it. As a simple example, if the only mage in the party is a sorcerer who you know has concentrated on almost exclusively damagey fire spells, and you throw a monster at them that is immune to fire, you have completely taken that spell caster out of the battle, and so a monster like this that would challenge a level 4 party that could to lots of non-fire damage, might outright wipe a party whose main form of magical damage is fire. Always read the whole statblock and all the effects, and consider what this will do to with your party.

This does not mean you can't, as an interesting challenge, present them with a monster who resists or is even immune to most of their effects. I threw an Iron Cobra at a party of 4th levels who had only one magic dagger and a few magic arrows (it's immune to non-magic weapons). They did have spellcasters, and eventually they beat it, but when the 2-weapon fighting dwarf critted with his axe and rolled awesome damage and I said "it bounces off with a clang and seems to do nothing to the snake", they got a little flustered and it took them a couple of rounds to figure out how to deal with it. Again that is fine, since I knew my party had enough other ways to deal with this snake... But if you had zero spell casters and zero magic items in the party, they would die to a thing like this because there wouldn't be any way to injure it. You have to know your party.

Answering your questions:

1. I'm not sure why they hit the Flayer for almost 0. It has a modest AC (15), and zero resistance to any weapons. Yes it has advantage on saving throws vs. magic, but tons of spells don't have saving throws, and there are no saving throws vs. swords or bows. So why couldn't the barbarian cut the thing's head off in a few swings? He should have been able to, unless something else was going on that you haven't told us. Had he dominated the Barbarian and turned him against the party or something? And why weren't the sorcerer's other spells effective?

2. I think a Mind Flayer Psion is a bit much for a level 4 party of 3 characters. The types of abilities they have are not super over the top but the players need to be able to deal with them. Again looking at the stat block... I'd put him up against my party of 4 level 5s now, and might even have to beef him a little (maybe a few more hp). But three of them at level 4? Probably not, unless they are VERY experienced players and the PCs, in character, already know what a Mind Flayer could do. Otherwise they wouldn't be prepared and someone's (as you have noted) going to have his brain extracted. You have to watch out for auto-kill abilities like this. There are tons of CR 8 or even higher creatures who can't do this sort of thing and only do straight hp (for instance, a Hydra is CR 8 but all it can do is bite you). Even most dragon's can't auto-kill you without you making death saving throws, and their CR is way above 8.

2, part 2. However, as a DM, you need to have a conversation with your players about how lethal the game should be. As a DM, we all find it hard to kill characters their players enjoy, but if you are afraid to do that, then eventually your battles will become boring and not tense, because the players will sense that death is not on the table, ever, and they can never lose. I have warned my players that death is on the table in my campaign. It hasn't happened yet, but they all know it is possible. Some have even made up back up characters just in case. You don't have to have a lethal campaign, and shouldn't if the players say they do not want it. But you should discuss it as a table, so everyone is on the same page. You might find they all say to you "We like our characters but we want death to be possible, don't feel bad" and you're clear.

3. Action Economy means that, barring special abilities or legendary creatures, each creature on the field of battle can only take one action per round. You can attack OR cast a spell (generally) but not both. The Action Economy means that the side with the weight of numbers has a huge advantage. That dragon may have a cool breath weapon, but he can only use it once, and then 6 or 7 characters get to go before he goes again (other than, as I said, legendary actions). The action economy means that a single monster, even a powerful one relative to the party, has a very short life expectancy, because it (usually) can't strike more than one PC at a time, and then the whole rest of the party can hammer it. Similarly, even a swarm of Kobolds can be dangerous to a higher level party if there are enough of them... because again, each one gets a shot, and they all have a chance to hit, and even slings can be dangerous if 20 of them are aimed at you and 11 of them hit for damage. Even at higher level 30 or 40 incoming damage in a single round is nothing to sneeze at. But the short answer to why a MF can't do what you are suggesting is that the rules say he can't.

If they had fun that is what matters but to answer your final question -- How I would have handled it would be:

(a) Not use another DM's adventure. If that means we can't play tonight while I prep something for next week then that's what happens. Let's play another game together for tonight like Talisman or Monopoly.

(b) Not use a Mind Flayer Psion against 3 level 4 characters. Its abilities make it a bit much for a small party like that to handle, as you saw. The fact that you had to forego some of its abilities so they didn't die proves it was too much.

Unfortunately, as you found, CR and experience points budgets may give you a general idea on monsters but as the game designers cannot see your party specifically, as a DM you have to learn by experience (sometimes, bad experiences; there's no help for it) what is too much or not enough or just right for a party. We tend to do this more intuitively, by feel, by what we have seen our party or other parties do over the years. There isn't any way for anyone else to give you that sense of intuition. You will have to allow yourself time to develop it over the months and eventually years (if you keep at it) and realize that there will be lots of times when you overestimate a monster's abilities, and some times when you underestimate them and there is a death or even a total party wipe. It's part of the game and to some degree you have to accept it.

To add a couple things to BioWizard's points, the party doesn't have to kill the monster. If things look bad they should have the option to run away. On the topic of character deaths, remember there are ways to revive a dead character. While I don't think anyone in this party has access to a spell like Revive, they do exist. So when designing an encounter, don't be afraid to kill a player character if their party has a way to revive them.

I’d like to add that sometimes it is better (makes more sense) for a monster to flee than stick around just to die. Same for player characters. You can also drag it out as a dramatic moment “I’d kill you now but the Mmmaster callllssss.” And then the big bad who just defeated the party exits stage left. 

edit — Welcome to being the Dungeon Master! You may end up stuck on this side of the screen!

All good advice, thanks for the input!

and yes ideally I would’ve made my first own session my own story, but as a starter I found finishing an existing story less daunting (I was wrong there). And I’d rather that I do this story than no one doing it and there being no DnD. This was still better DnD than the old saying “bad DnD is worse than no DnD” xD 

It's understandable. And it definitely would seem, if you hadn't any DMing experience, like using another DM's story and adventure would be easier. It's just that, for those of us who have a lot of experience, we can tell you that it's really not (as you saw).

True, after the current arc I’ll hopefully be able to play my own written story where i alone can determine which direction I want the story to go: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/dungeon-masters-only/84451-need-help-checking-my-story

First, welcome to the club :) .. if you and the players had fun then you were doing your job :)

Just a couple more comments ...

1) Ghosts can be nasty against a small low level party. At level 4, the characters just don't have the spells or resources to deal with possession, other than reducing their friend to zero hit points and forcing out the ghost ... all while the ghost keeps hitting them while possessing the party member. In this case, I would probably just have "forgotten" to use the possession ability. A Ghost may only have 45 hit points but it is resistant or immune to most of the damage types the party will be able to use making it a much more challenging opponent than it would seem. In addition, it does an average 17 damage/hit (4d6+3) while most level 4 characters will have an average of about 30 hit points (d8 with 14 con) which means that any of the characters (except the barbarian) in the 3 person party could be on the ground after 2 rounds. Since the ghost can fly, move 40' and pass through solid objects - there is a good chance it might ignore the barbarian for the squishier characters hiding at the back.

Finally, if there was more than one ghost ... it would be a deadly encounter (according to kobold fight club) and the characters might have a lot more trouble.

2) In the case of the mind flayer psion - it is a CR8 with 5th level spells - against 3 x 4th level characters - it is a deadly or more encounter and if played properly, the odds are good that the party would not survive (though that depends to some extent on their equipment including the number and type of magic items).

This is where it comes around to Biowizard's comment running another DMs encounters  ... it can be difficult for an experienced DM to figure out encounter balance and much more difficult for someone who hasn't DMed much. It is hard to tell just from a Roll20 setup whether the DM planned a combat or social encounter. Was the party meant to fight the psion or parley with it? If they were meant to fight, was the psion already down spell slots? Fear, confusion and telekinesis could all mess up a party with only 3 level 4 characters. Even phantasmal force could potentially keep one of the three occupied with an imaginary opponent or crown of madness could have one of the characters attacking the party.

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1. Against the mindflayer, cantrips and regular weapons should work fine so although they would do more damage with spells the party should still be hitting. AC15 with a +5 to hit gives a 10 or more to hit and roughly half of the attacks should be hitting. Bad luck will cause misses and with only one attack or cantrip/turn it will take quite a few rounds for only 3 characters to get through 71 hit points but it can be done if they survive (which isn't that likely). Scorching ray does 21 hit points on average if all the bolts hit but at that level the odds are that only 1/2 do so the average is about 11 damage/spell ... which is still less than 1/2 the target's hit points after 3 rounds of combat.

2. In my opinion, the mind flayer should have wiped the party between tentacles, brain eating, mind blasts, stuns and spells ... the only way the party can survive is if you don't use most of its abilities which is what it sounds like you did ... which is probably the best move in this sort of situation where a DM puts the party up against something that they clearly can't handle.

3. Monster action economy is exactly the same as the players. They have one action ... they have bonus actions and reactions if something grants them those abilities. The things they can do with their action are listed in the monster stat block. So a mind flayer can't use their tentacles to grapple something and cast a spell in the same turn since one uses the attack action and the other the cast a spell action and they can't do both unless the stat block says so or the DM decides to modify the monster stat block to allow them to do so.

Similarly, if the monster has legendary or lair actions, the stat block will say so.

In this case, it means the mind flayer has to choose between casting a spell, using its mind blast, grappling something with tentacles or trying to eat a creatures brain.

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For the encounters given, I think you did a fine job making the changes needed to give the party a chance. The only real change I would have made would be to not necessarily run these encounters as written since they may have been intended for 4 level 4 characters and not 3 so they might have needed some adjustment for the reduced number of characters to get the right level of difficulty.