Teach by showing
Many people don't want to learn things that will not benefit themselves. One easy way to show them the benefits without spoonfeeding them is to have NPCs do the thing you want the players to learn.
For example, have the NPCs fight a group of goblins. Have the goblins force the players to run away or re-position constantly. The players will quickly learn that getting hit by opportunity attacks sucks. Have the goblins constantly run away and re-position, but have them always disengage to avoid being hit. Be open about the way the goblins are behaving; "this goblin takes the disengage action to avoid triggering opportunity attacks, and then moves to here".
Present a situation where the mechanic is important
Often in D&D players will face encounters appropriate to their skill level. This prevents players from learning. You should consider having them face encounters that they cannot beat at their current skill level. Give them an encounter that they will not win unless they use the mechanics you want them to learn. Just make sure you first show them the mechanic with NPCs so they are aware of it.
Give them the opportunity to discover and apply the knowledge. You may be surprised, they may understand the mechanic but not think it is useful because the fights are too easy, or it's more fun to do something else.
Disengage is a great example of this problem. Most classes have to give up an attack to disengage, so I think many players will think "well, I can just attack and try to end the fight sooner rather than running and being hit next turn anyway".
Accept the player's weaknesses
Be aware that learning may be a slow process. It may take several fights against different species and classes to get the idea across.
It's possible that even with an incentive your players still won't want to learn. That's fine, they enjoy other parts of the game and their combat will suffer. There are plenty of people who play D&D for the combat and are poor role players. Does it really matter if the party aren't great in combat?