How to help players who don't know how to play their class?

I'm running a short campaign, and several players don't seem to understand how to use their class strengths. The Barbarian forgets to use his Rage half the time; the Rogue doesn't know how her Sneak Attack and Cunning Action work and the Bard doesn't realize that they aren't meant for close-quarters combat against a strong monster.

These players usually take twice as much time on their turns as the other party members trying to figure out what to do. They usually end up asking me for a rule clarification before they finally make a decision. And even then, they still don't think very tactically.

In the last session, the Bard got killed while the Barbarian stood beside him. Instead of using the Help Action, dragging him away from the monster, or even trying to distract the monster, the Barbarian instead chose to keep hitting another monster that wasn't threatening anyone. I'm not even sure the Barb knew the Bard was in danger.

Like, I don't want to be mean. I'm fine reducing the difficulty of combat, but I don't know if they're enjoying this. I would imagine playing Barbarian or Rogue without using Rage and Sneak Attack, respectively, isn't very fun, and the Bard seems to be having a pretty miserable time overall. (I don't think he even knows how Bardic Inspiration works).

What do you think I should do?