After years, they still don't get it
My group has been playing for 3 years. Most everyone was new except the first DM and the current second (they swapped after the first campaign). There are 2 sisters that have been with us from the start and they... just don't seem to get the rules. They do things like forget which numbers to add to dice rolls and forget their own abilities. Last night, one of them who played the Barbarian last campaign, she's playing the Ranger now, we told her to use her bonus action for Hunter's Mark. She argued back she had already used her action to stealth, until someone finally realized the confusion and explained the difference between action and bonus. She said she didn't know she also had a bonus action. We've been playing for 3 years. And 2.5 of those years she was the barbarian using her bonus action to rage. You might think, maybe she's still getting used to the new character? But no, this wasn't our first combat.
How do we help them? The first DM took an approach of patient explanation mixed with I'm going to let you make mistakes and explain them later so you learn. The current DM is a little more abrasive and up front with saying no that's a bad idea and calling them out for not knowing things. We the rest of the players try to pick up the slack and help answer their questions and make suggestions asking the way. But like what advice do you have for helping people remember the basics?