>>199I'm way late, but this board is neat and this question very important.
Firstly, "looking for answers" is definitely not why anyone should dabble into philosophy. "Looking for ways to answer" would be more appropriate. "Looking for questions" even more so.
And there you have it. The instant someone claims to have understood something to the extent of feeling comfortable turning it into a factual statement, they need to be more than capable of backing up every single step that lead them to that conclusion. Starting from the premiss, all the way to their conclusion, everything needs to be sound.
If at any point during his argumentation you disagree with him (with reasons of your own) you are obliged to ask him to either back his claim up some more, or to consider and refute your counter-claim. If he can't do either of that (to your satisfaction), he miserably fails as a teacher and as a philosopher.
This has nothing to do with truth, but everything to do with understanding of arguments. It shouldn't be that you dislike or disagree with his conclusion, you need to find a flaw in his argument for that conclusion, be it a simple but disputable premiss. It's not that he's "right" or that you are "right", it's that he clearly considers his argumentation to be so flawless as to surpass any alternative. In almost any philosophical context, such a claim is impossible to hold. And all you need to do to show that is to ask your teacher to back up his premiss or premisses against your counter-claim.
As for your second question; Try to understand his reasoning regardless. Even if you feel like you've completely grasped it, and you still disagree vehemently with his take on the problem, you have understood and followes a thought process that was not your own original one. Doing that, being able to do that fairly effortlessly, is absolutely vital to philosophy. Unfortunately it is not very common to cultivate this.