People can have trouble with the rules because they can’t afford the PHB, or don’t want to shell out for it until they know they actually like to play, or just don’t feel comfortable looking things up in the middle of the session. Sometimes the rules just don’t make sense to someone until they’ve seen how those rules apply during gameplay. I was extremely embarrassed the first few times I had to say “this rule doesn’t make sense to me, can you give me an example?” but sometimes it just takes the right kind of explanation to make it click.

Assuming you have tried different ways of explaining and are pretty sure it’s a memory thing, you could suggest that if a player doesn’t have the PHB, they should not play classes that rely heavily on game mechanics. You could also try giving the confused players copies of just the pages that you think they need, and making it clear that you don’t care if they look things up during a session, provided you actually don’t care. If you really need a group to be good on rules without looking anything up, it might be a situation where you have to figure out how to tell a player to find a different group.