We ask What...? when there are a lot of possible different answers. Example: What sport do you play? (There are many sports.)
We ask Which...? when the answers are quite limited. Example: Which way should I turn? (You can turn right or left only.)

If we include the possible answers as part of the question, then we use Which. Example: Which restaurant did you go to last night? 'The Hot Potato' or 'Pine'? (There may be many restaurants but in the question we are giving only two choices.)

If we don't include the options, the answer may be any restaurant, and that's why it should begin with What. Example: What restaurant did you go to last night?

Preguntamos What...? cuando hay muchas respuestas diferentes posibles. Ejemplo: What sport do you play? (Existen muchos deportes.)
Preguntamos Which...? cuando las respuestas son bastante limitadas. Ejemplo: Which way should I turn? (Puedes doblar a la derecha o la izquierda solamente.)

Si incluimos las respuestas posibles como parte de la pregunta, entonces usamos Which. Ejemplo: Which restaurant did you go to last night? 'The Hot Potato' or 'Pine'? (Puede haber muchos restaurantes pero en la pregunta estamos dando s髄o dos opciones.)

Si no inclu韒os las opciones, la respuesta podr韆 ser cualquier restaurante, y por eso debe comenzar con What. Ejemplo: What restaurant did you go to last night?