I'd love to hear Alfonso's explanation as well, but I'll give it a try.
There are a few rules that will help you know when to use the interrogative pronouns
¿qué? and
¿cuál?.
1. Generally,
qué = what and
cuál = which, but there are some caveats.
2. Always use
qué if a noun comes right after it: (Whether or not you would use
which or
what in English).
Qué is used as an adjective here.
¿Qué regla es la que debo aplicar aquí? - What rule should I apply here?
¿Qué cosa? - What?
¿Qué color te gusta más? - What color do you like better?
3. Use
qué to ask for an explanation, description, definition etc, when not followed by a noun: (
qué is a pronoun here)
¿Qué es esto? - What is that?
¿Qué significa esa mirada? - What does that look mean?
4. If you use 'what' and you can substitue 'which' or 'which one(s)', then use
cuál. Of course, using which or which one might sound stilted in English, but you can say
which is the topic for today?, but you could never say
*which means that look? Note that you're asking for a specific thing (regla, tema) instead of asking for an explanation, description, or definition as in #3.
¿Cuál es la regla? - What is the rule?
¿Cuál es el tema de hoy? - What is today's topic?
¿Cuál de los dos me queda mejor? - Which one fits me better?
¿Cuáles son las notas de esta canción? - What are the notes for this song?
5. Note that
cuál is never used with a noun right after it.