Am I allowed to put a physics problem that pertains to a certain subject, and answer that problem so other people can understand the concept? I think doing this would help other people so that they won't be confused with what they're doing.
We don't have a rule against problem explanations specifically, but I think there's a decent chance these sorts of questions would run afoul of our homework policy. You don't get any special consideration because you're the one posting both the question and the answer; in other words, your question and answer will be examined for compliance with the homework policy the same as if they had been posted by different people.
It really depends on how you ask the question, though. If your question asks about a concept, and presents the problem only as an example of how the concept might be applied (rather than asking for the answer to the problem), it might well be fine. And if your answer only shows how to apply the concept in the context of the problem, rather than giving a full solution, again, it could be fine.
I did something very like this in How long would it take me to travel to a distant star?
Asking how to calculate the travel time to a star could be a homework problem - I bet it's been set as course work many times. However in this case the emphasis is on understanding the physics involved rather than just giving an answer. If you were thinking of something along these lines then I'd say give it a go. You can always delete your question and answer if it's not well received.