Concerning the question whether que or qual / quais is to be used there is no difference between Portuguese and English, however the difference between which and what is sometimes a little bit subtil. The difference between "What do you want?" and "Which do you want?" is obvious. In the first case we ask for something unknown, in the second case we ask for an element of a group.
In the case qual / quais is used as an interrogative adjective it not so evident, the difference between "What kind of music do you like?" and "Which music do you like?" is not so evident and the answer for both questions could be the same.
In the case of an adjectival use we can therefore distinguish between three different situations.
1) For which book Thomas Mann got the Nobel Prize? => We ask for an element of a group. The books written by Thomas Mann.
2) What kind of person is he? => We ask for properties: nice, intelligent, rich, etc..
3) What kind / Which paintings do you like? In both cases we can answer with the paintings of Picasso, the funny ones, etc..
The same logic is obviously true if qual / quais is combined with a preposition. In case of an interrogative pronoun the difference is obvious: What you are talking about. <=> About which you are talking about. In case of interrogative adjectives there is an intersection where que as well qual / quais are possible.
It is possible, although not necessary, to put an o in front of que. The o doesn't have any semantical value, que is the same thing as o que: "Que queres fazer?"<=> "O que queres fazer?"
Choose in the following sentences the possible options.It can happen as well that two options are correct, but the meaning is different. In this case choose all possible options. Justify your choice. (The English translation is ONE otpion, what doesn't mean that it is the only option.) You find an explanation by clicking on the eye.
0) Tens alguma noção do -- queres fazer?
Do you have anyy idea what you want to do?
(o) que
a qual
as quais
1) -- das duas afirmações é verdadeira.
Which of these two assertions is true?
(O) que
Qual
Quais
2) -- fique incompleto, é isso que queres?
You want that it remains incomplete, it that what you want?
