"What do you not understand?"
Translation:Kion vi ne komprenas?
It's comprehensible but „La KI-vorto normale staras komence de la demanda frazo“ (see: http://bertilow.com/pmeg/gramatiko/demandoj_respondoj/ki.html)
-i is the infinitive, or "to …" form. To do, to be, to eat, to learn, etc.
-u is the imperative or command. One mnemonic is, "just -u it! "
-us is the conditional. Used for would/should/could and if…then… sentences. For me, -us sounds close enough to the -ould words to help me remember it.
-is/-as/-os are past/present/future. My mnemonic is a visual image of a road with the vowels on it, going from a tiny point in the past (i) thru the medium width of present (a) and widening into the future (o).
Hope that helps.