"What do you not understand?"
Translation:Kion vi ne komprenas?
Because it represents the object in the sentence. If the thing not understood was leaves, we might have said "vi ne komprenas foliojn". The accusative in "kion" exists for the same reason as the one in "foliojn" - it's the thing the action is acting on.
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 could use examples of komprenas, kompreni, komprenis, komprenos, komprenu, and whatever else there are.
I never liked hearing about tenses, cases, moods, etc. in English, and what I understand while I reading the explanation is gone the next day.
-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.