¿Quieres comer? ¿Quieres comen?
Why can't use comen here? Thanks.
Because comer is the infinitive (the "to" form of the verb), its the same in English, you would say "What do you want to eat?" You second example would translate as What do you want, they are eating? and would make no sense. Hope that helps
It is a good thing that the second verb is the infinitive (the 'unchanged' verb) because after you use queres or other verbs (including "voy a") then you don't have to think about it. You just follow it up with the unchanged verb. It does get a bit confusing if you use the subjunctive. If you want to say "What do you want me to eat?" or something then in English, we often miss out the "that" bit. We don't say "What do you want THAT I eat?" but you do in Spanish: ¿Qué quieres que yo coma? = What do you want THAT I eat? If you said ¿Qué me quieres... well... me quieres means you love me. The subjunctive is a bit of a difficult tense but like most tenses, there are plenty of patterns. If you get that far, try reading http://spanishplus.tripod.com/PresentSubjunctive.htm#TopOfPage