How to conjugate verb in korean?

I know that in korean, conjugation is not coz by the subject. It's coz by the sentence formality/informality. And i know that you should drop the -da in the end and put the right one to be conjugated. My question is, what will i put to the end after i drop the -da? Like the verb jiuda will become jui-, . .jui what? Pls don't write it in hangul. :-)