I don't understand why/what/when/where/which and how, why they change forms, and how to use them. I just don't fucking get it.
If you mean the adverbs warum (why), wann (when), wo (where) and wie (how), and the pronound was (what), they shouldn't be changing forms. As for which, what exactly do you have in mind? 'Welcher' and its declensions? Or do you mean der, die, das etc, used as relative pronouns?