There is a question for how to help another player remember the rules. We have a slightly different problem: our DM is forgetting how even our most commonly used character features work, and is constantly perplexed how we get to our attack bonus or damage, or whatever it may be that works in our favor, and keeps asking about it.
For example, we have a rather overpowered Gloom Stalker ranger, and practially every session when he uses a feature like Sneak Attack, and tells him the damage, the DM is incredulous and asking questions about it. Then, we have to walk him through how we got to that number. We are all old friends, and so the tone in those situations can get a bit tense at times "Come on, you ask that like every effing time! How can it be you cannot remember this, he's using it all the time!". It may not help that we all have busy work and family schedules, and the cadence of play can vary and often is at best bi-weekly; nor of course, that our 15th level characters have a lot of features and items stacking up by now.
Scolding the DM does however not help them to remember better. So we have the same conversation three weeks later again. Has anyone else had this problem and found an approach to help address it?
In case it is useful: we usually play online on Roll20, with voice via skype. We are playing D&D 5e, but I felt that you could have this issue with any RPG that has moderately complex character ability structures and rules, so I don't think the issue is specific for 5e, and did not tag it as such.
Best Answer
Set macros so that every roll is self-explanatory.
That's a lot of work (especially on roll20), so you don't have to do this for every feature, but doing it for the worst offenders could only help.
On roll20, when someone rolls something stuff appears in the chat (the total result of this roll, but also the result of each individual die). You can create your own macros so that relevant details also appear as comments. This could display something like this for one of Mr Gloom's attacks:
Configure it so it stays up-to-date when you level up or acquire other modification and your DM can simply give a look at the chat every time they think the amount of damage is suspiciously high.
I personally use this as a player when I am playing Pathfinder 1st edition (who isn't nicknamed Mathfinder for nothing) as otherwise I wouldn't know which bonuses I already accounted for, but I have to admit roll20 doesn't make creating and using those macros the easiest thing. It is still manageable though.