"Czego się pan napije" meaning

Why do you use czego? Isn't that the genitive form? Why not co? Also what does się napić mean? How does it differ from pić and wypić?

"What are you having to drink?", would I suppose be the most suitable and literal translation. The"sie" of the reflexive form of the verb "napic" > "napoj" = drink, beverage, is the equivalent of the English "one", e.g. "How does one.. aka "you"...?" etc.

Many verbs in Polish require certain cases which seem a complete mystery to Anglophones. The genitive is simply part of daily usage in any number of situations. "Potrzebowac", "szukac", "napic" etc.all require the genitive, as do ALL verbs (except for "byc" = to be) expressed in the negative, for instance:
Accusative - Bardzo lubie polska kielbase. vs. Genitive - Nie bardzo lubie polskIEJ kielbasY.

There are many such instances of the genitive in Polish, in fact, it's the most widely used case in the language, including after all wishes such as

"DobrEJ zabawY!" = Have fun! Have a good time!

SzczesliwEGO NowEGO RokU! = Happy New Year!

SmacznEGO! = Bon appetite! Enjoy your food!

and so forth.......

"Czego" is the genitive form of "Co" or "what".

Co to jest? = What is this? (Nominitive Case = naming case, stating what something/-one is called)

"CzEGO szukasz?" = What are you looking for? literally "Of what you look?" (Genitive case = case indicating, among other things, possession or belonging to something/-one, "of" someone-(thing) etc......)

A little clearer now?

Good question while we not say czego się pan naje ;)
but Czego się napijesz? and Co zjesz na obiad?

I find it logical that some verbs just need genitive and so on. But what does napić się mean? Why not pić or wypić? I only found a Polish web dictionary about it but it was too hard to understand.

"Czego się Pan napije?" is the most natural, but we may also say "Co Pan będzie pił?".

@OssiKabanossi
Wypić means to drink all ie empty the glass/bottle
Pić means the act of drinking is continuous and not finished
Napić means to drink some

The prefix/preposition "na-" in Polish can sometimes indicate the completion of an activity, as in "pic" vs. "NApic", "pisac" vs."NApisac" etc.....
This belongs to one of numerous hurdles we foreign-born students of Polish must overcome.

Correct aspect use along with verb conjugation however, is said by experts (among them Dana Bielec) to be the most frequent cause for errors by foreigners (even Poles too) in mastering the Polish language:_)

@Lyzko if napic is perfective, what is the difference with wypic?

Both mean "to finish drinking", although as I've heard both used, the former points more to the act of drinking to completion, yet not within a fixed interval of time. A reasonable period would be then roughly from the time one starts to drink up until the time one has finished the drink. The latter one you mention is almost like finishing a drink practically in one shot aka "down the hatch"! Both designate completion, but one is continuous completion, the other isn't:-)

Does that make sense?

Wish you knew some German as well, because such prefixed verbs are part of the DNA of the language, far more than in contemporary English!!

I should add that ALL "wy-", "na-" verbs are ALWAYS going to be perfective, their usage depending often on context as in my above explanation.

That makes it clearer, thank you.

@Lyzko Talking about German did you mean austrinken?

I certainly am:-) In the latter, if someone says, "Trink mal aus und lass uns gehen!", what is meant is doubtless much the same as "Wypij a idzmy!", cf. English,

"OK, drink up (vs, the common 'Germlish' confusion, "Drink OUT".....!"LOL) and let's go!"

:-)

Merely wish to add that "napic" is ALWAYS going to use reflexive "sie" + genitive, while "wypic" requires neither:-)