There’s a player in my party who consistently confuses themselves when it comes to their Paladin’s abilities. The DM and I do our best to gently explain how their class works, but it seems like there is an issue every session. How do we help them overcome? When I've had that problem it usually meant the player wanted to play a simpler game/character. I'd try and figure out what they want from their class, and find something simpler that did it.
— "Matt Colville?" (@mattcolville) July 3, 2019
In my experiences, taking ten or twenty minutes to run through “Basic Combat Drills” can help a confused player learn their character’s abilities.
You give them hypothetical example situations, and quiz them on how their abilities work, to troubleshoot problems in advance of actual play.
Also consider Spell Cards and Ability Cards, which let them hold the rules for their abilities in their hands and double check them as needed.
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DM: Let’s say your friend is wounded and needs healing. How can you help?
PC: Uhh… I can cast Cure Wounds, right?
DM: Correct. What do you need to cast that?
PC: Uhh… I need to use a 1st level spell slot.
DM: Right. You can also choose to use a 2nd level spell slot instead, or a 3rd level one, and so on. If you do that, the spell is stronger and it gives you extra healing power. Now, what else do you need to cast Cure Wounds?
PC: Uhh… I need to use an Action?
DM: Yes. When can you do that?
PC: Umm… on my turn, I guess?
DM: Exactly. You can’t use Cure Wounds your friend’s turn, only on yours.
PC: Okay…
DM: How about spell components?
PC: Uhh… I have my Holy Symbol, right?
DM: You do, but what does a Holy Symbol let you do?
PC: Doesn’t it replace spell components?
DM: It replaces -material- components, like a pinch of dust or whatever. But it doesn’t replace verbal components, which are the words you speak, and it doesn’t replace somatic components, which are things like gestures you make with your hands. Check your spell card – what components does Cure Wounds require to be cast?
PC: Uhh… verbal and somatic.
DM: So if your character loses their voice, can you still cast the spell?
PC: No.
DM: And if you get your hands tied up, can you cast the spell?
PC: Also no.
DM: So to cast Cure Wounds, you need 1) a spell slot, 2) an Action on your turn, 3) you have to be able to speak, and 4 you need at least one free hand.
PC: Okay.
DM: That’s not all. There’s one more thing you need. Any idea what?
PC: …uhh… I dunno…
DM: What’s the spell’s range?
PC: Oh! Right! Uhh… Touch. So I have to touch the person I want to heal?
DM: Bingo! You need a spell slot, an Action, the ability to speak, a free hand, and you have to touch your friend to cast Cure Wounds on them.
PC: Cool. Got it.
DM: So then what happens?
PC: Uhh… they make a saving throw? Wait, no… umm… they heal 1d8 plus my spellcasting modifier.
DM: Right. What’s your spellcasting modifier?
PC: Err… that would be… my…. Charisma?
DM: Correct. How much do you add from your Charisma bonus?
PC: Uhh… a 16 Charisma is…. +3?
DM: You got it. So how much do you heal them for?
PC: Uhh… 1d8 + 3 hit points.
DM: Perfect. You got it.
PC: Cool. That’s not that hard.
DM: Right – now let’s say you were out of spell slots. Do you have any other ways to heal your friend?
PC: Uhh… I have… is it… Lay on Hands?
DM: Yep! Now, how does Lay on Hands work?
~~~
Etc, etc, etc.