(War caster feat, second gift: You can perform the somatic components of spells even when you have weapons or a shield in one or both hands.)

So, without this feat, you can't cast any spell with a somatic component while one or both hands are holding a weapon and/or shield? Why?

Why cant you do spells one handed, like the somatic for fire bolt or magic missiles being a finger gun? Why do you need to use your hands? Couldn't you use face twitches to shoot Eldritch blasts out of your eyes?

And what about the pact of the blades improved pact weapon? It is a weapon (sword for this example) but it says it counts as a spell focus (something you can cast spells out of, like a wand). So does that mean l can use it to cast spells, even if l don't have war caster? Or does that mean that l could use it to cast spells, but only after getting war caster?

Could the dm just hand wave it? sure, but l want to know if there is a offical ruleing on my questions.

In other words, If l had a warlock Hexblade pact of the blade, with improved pact weapon, could l cast spells with my pact blade in one hand without the war caster feat? if yes, could l still do so while holding a shield in my other hand, or would that require War caster?

 (ok, so l think i understand. thanks)

From the rules

"Casting some spells requires particular objects, specified in parentheses in the component entry. A character can use a component pouch or a spellcasting focus (found in “Equipment”) in place of the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell.

If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell. A spellcaster must have a hand free to access a spell's material components -- or to hold a spellcasting focus -- but it can be the same hand that he or she uses to perform somatic components."

If your sword or shield are not a spell focus you need a free hand to cast spell with either a material or a somantic component

An improved pact weapon can be used as a spell focus if the spell spell has a material component the warlock can use their sword to replace the material component and use their sword had to perform the somantic component. If the component has a value you must have that component. The rules do not say "you can use a  spellcasting focus  in place of the components specified for a spell unless a cost is indicated for the component" so I would interpret that to mean if you want to cast summon fey you can do so using your sword as a fous as long as you have a gilded flower on your person.  If the spell has no material component however the sword is not being used in place of the compnent. In this case to carry out the somantic component you need a free hand and can not use the hand holding your sword/spell focus.  One interpretation I have heard for this is the somantic component for spells thet use materials are quite vague and can be done with a hnd holding either the material itself or a focus but a spell that uses only a somantic (and possibly vocal) compnent is much more intricate and needs a completely free hand.

Clerics and Paldins using a shield as a spell focus are in the same situatiuon

ok, l understand the spell focus replaceing non costly components part. that isn't what l was asking. however, "A spellcaster must have a hand free to access a spell's material components -- or to hold a spellcasting focus -- but it can be the same hand that he or she uses to perform somatic components." does (if l'm reading it right) mean that if im holding a sword, but not a shield, casting a spell shouldnt be a problem, cause l have one hand free. Right? And that, because you can use a spell focus (like a wand) to cast a spell, if your sword is also a focus, you can also cast a spell while holding it, even without war caster?

Quote from DnDNewper >>

ok, l understand the spell focus replaceing non costly components part. that isn't what l was asking. however, "A spellcaster must have a hand free to access a spell's material components -- or to hold a spellcasting focus -- but it can be the same hand that he or she uses to perform somatic components." does (if l'm reading it right) mean that if im holding a sword, but not a shield, casting a spell shouldnt be a problem, cause l have one hand free. Right? And that, because you can use a spell focus (like a wand) to cast a spell, if your sword is also a focus, you can also cast a spell while holding it, even without war caster?

If one hand is empty you can cast anything you want (assuming you have the required materials on your person). If you are holding a focus you can cast any V, M, VM, SM or VSM spell (except if it has a gp cost or consumed material). If your hands are full of weapons or shield (even if one is a focus) then you can't cast any S or VS spells, because you need an empty hand. You need War Caster to cast those S spells without free hands. 

Personally, I only apply those rules when the focus is a weapon or shield. I consider a hand holding a wand or crystal (etc.) focus to be empty enough to cast S spells, though this is not supported by RAW. 

The RAW regarding foci and somatic components are very non cinematic. If Gandalf were following the rules, the Balrog would have eaten him on round 1.

The way I play it is if you have your focus in your hand whether it is a wand, staff, weapon, and/or shield cast whatever you want. For my table it ruins the vibe when you tell the caster they can't cast a spell because they have their wand in their hand. Really?

If there is no material component but there is a somatic then you have to have a free hand thus needing to put away the wand (or whatever).

Yay, this topic again. Let's hope this one doesn't go to 20+ pages (or was it 40?).

As for "why are the rules what they are" questions: I don't know, doesn't matter, that's what the books say.

Now for material/focus, somatic, and war caster breakdowns:

1. You must have an empty hand to perform somatic component unless the spell has a material component you are using the same hand for.
2. You must be able to hold or interact with your focus or material for material component spells.
3. War caster lets you ignore 1 and perform somatic components with a hand that is holding a weapon or shield.

Typically, if you have 1 hand empty, you are good to go. But if you have a special focus, a weapon, and/or shield you may want to consider this feat to avoid juggling items (and just because it is a good feat for any caster in the fray).

Quote from DnDNewper >>

Why cant you do spells one handed, like the somatic for fire bolt or magic missiles being a finger gun? Why do you need to use your hands?

It's for game balance. You have to make a choice; a choice that has consequences.

Want to attack? Need a hand to hold a weapon.
Want to defend? Need a hand wearing a shield.
Want to cast a spell? Need a hand to make gestures and/or manipulate components.

Want to do all three? You can't. Make a choice. 

Or just use a component pouch and miss out of every other time you may get to use your reaction to make a weapon attack.

Edit: there are spells that are reactions too. But essentially only reactions are affected.