The interrogative "cuál" is always 'pronominal', that is, it is NEVER followed by a noun, although a noun is always 'understood'. If you explicitly supply the noun, however, the result is always ungrammatical: *Cuál libro/vino/película/profesor/asignatura/ciudad... te gustó más? etc. are all bad; another way to explain this is to say that "cuál [ ]" is always elliptical, with the omitted noun [ ] ranging over a DEFINITE (= contextually defined) set of INDIVIDUALS/SUBSTANCES. Thus, "¿Cuál es mejor?" is always understood as 'Which of the individuals/substances [of the contextually defined class you and I are talking about] is better?'. On the contrary, "qué" admits of both 'pronominal' and 'adjectival' uses. When "qué" is pronominal, it is NOT followed by a noun, its reference is INDEFINITE (there is no range of values previously agreed to by the speaker and the addressee), and the reference need NOT be an individual; it may be anything: an individual, a substance, a property, an event, or even a proposition. For example, the answer to "¿Qué quieres?" may be "una bicicleta", "más dinero", "irme a la India", "Que me subas el sueldo"; the answer to "¿Qué dijo?" may be "No", "Nada", "Hola", "¡Que te follen!", or "Que no tiene tiempo", etc. The answer to "¿Qué te parece?" may be "Bueno", "Bien" (properties), "Un idiota" (a predicate), "Que es bueno" (a proposition), etc.= "qué" ranges over just about anything nameable! When "qué" is adjectivally used, on the contrary, it must, by definition, be followed by a noun that determines its range of values, but, the noun does NOT refer to a set previously defined by consensus between the speaker and the addressee, and again, it need NOT refer to an individual: in "¿Qué coche prefieres?", "coche" denotes an individual, but in "¿Qué tiempo hace?" "tiempo" does NOT refer to an individual, in "¿Qué años tiene?" "años" refers to a numbered set, etc. In sum "qué + noun" means "what + member of the set the noun names", but nouns may name universal, or even quite 'wild' sets!

I understand you are talking about "cuál" and "qué" question words. For me it is very difficult to explain the difference but i'm going to try it. Here you have a very simple way to explain it: You use "cuál" to choose something of an homogeneus group: For example you are seeing a group of suspected people of committing a murder and the policeman says: - ¿Cuál es el asesino? (in a group of suspects) Or you want to buy a pencil but you have a lot of them to choose and you ask: - ¿Cuál escribe mejor? (in a bunch of pencils) Other example: - ¿Cuál es tu nombre? (talking about all the names of the universe :P) You use "qué" to choose something of an heterogeneus group and almost in the rest of cases: - ¿Qué prefieres, un libro o una película? - ¿En qué trabajas? - ¿Qué harás el próximo fin de semana? Be carefoul, You can't always translate directly what/qué nor which/cuál

Mario, muhcas veces coincide, pero no siempre, por ejemplo: - What's your name? -¿Cuál es tu nombre?

big difference cual = which que = what