Which one is correct:
1.It is you who doesn't understand, or it is you who don't understand?
2.It is you who need help, or it is you who needs help?
3.If I were you, I would never do that, or if I were you, I would never have done that?
4.I am disappointed in you, or I am disappointed by you?
I'm a native English speaker, and yet I found myself thinking why "It is you who doesn't understand" sounds correct. It is correct, though. If I replace "you" with "me", the phrase "It is me who doesn't understand" sounds a lot better than "It is me who don't understand". The rule is that the verb takes the third person.
However, the sentences "You don't understand" and "I don't understand" are both correct.
Here's another set of examples.
It's me who is here, It's you who is here, it's him who is here!
But it's us who are here, it's all of you who are here, and it's them who are here.
For item 2, it's the same pattern, so the correct sentence is "it is you who needs help".
Item 3: both are right, but in different contexts!
"If I were you, I would never do that" refers to an action that does not take place at one particular time or that could take place several times. For example, "If I were you, I would never sing in public" implies that I should not sing in public now or later.
In contrast "If I were you, I would not have done that" implies that the action took place in the past. So, for example, "If I were you, I would not have broken up with him" means that you did break up with him, but I would not have done so.
Item 4 - the correct one is "I am disappointed in you", though "I am disappointed with you" is almost as good. If you are unsure, type in Google the phrases in parentheses, and see which one gets the most hits.