"I'm helping him understand" might sound good if you write what it is afterward. Like "I'm helping him understand how to do this math problem." Without the context it sounds a little off. "I'm trying to make him understand" sound like you are very frustrated. I could imagine someone in a failing relationship saying something like this. "I'm trying to make him understand my point of view, but I just can't." Making someone do something usually sounds quite forceful. A similar meaning might be "I'm trying to get him to understand..." which sounds more like you will guide this person along trying to understand. I don't really understand the sentence "I'm forcing him and making him understand." Maybe it sounds like you are frustrated trying to help a lazy friend study for a test. You try to study with the friend, but he doesn't want to. Somebody asks "What are you going to do?" you could say "I'm going to force him to sit down, and I'm going to make him understand!" Basically you should write what you mean, instead of leaving information out. Forcing him to do what? Making him understand what? Then it should be good!

No, it does not sound right to say, "I'm forcing him and making him understand." I hear "I'm forcing him" and I wonder, "forcing him to do what?" and I sort of expect to hear "I'm forcing him to ________(<-- verb of some kind)." "Forcing" and "making" also seem redundant -- two verbs that mean the same thing in this sentence (why would you need both?). I think the verb "making" works better than "forcing" in this sentence. I'm helping him to understand. yes I'm trying to make him understand. yes I'm making sure he understands. yes I'm forcing him to understand. meh. The grammar is OK, but it isn't how we would usually say this. I'm forcing him and making him understand . no I hope this answer was clear and helpful.

Nothing is wrong.