what: ✓ what ✗ which

what
which

I can’t tell you what your purpose is. That’s got to come from you.
Non posso dirti qual è il tuo scopo. Quello deve venire da te.
– Adrian Grenier

✓ I don’t know what subject to study at university.
✗ I don’t know *which* subject to study at university.
Non so quale materia studiare all’università.

Basically, we use what + noun to refer to a relatively unlimited number of possibilities, and we use which + noun to refer to a relatively limited number of possibilities. Continuing from the example given above, notice that if the choice is between two subjects to study, we use which:

I don’t know which subject would be best for me – law or economics.
✗ I don’t know *what* subject would be best for me – law or economics.
Non so quale materia sarebbe meglio per me – giurisprudenza o economia.

Some more examples:
What‘s your favourite colour?
*Which* is your favourite colour?
Qual è il tuo colore preferito?

We use what in this question, because there is no limit to the number of colours. Compare this with:
Which (one) do you prefer – the blue one or the black one?
*What* (one) do you prefer – the blue one or the black one?
Quale preferisci – quello blu o quello nero?

The number of answers to this question is limited, so we use which.

Now try the quiz below! There are TEN answers. The pass mark is 100% (10 out of 10). Click on the blue arrows to go to the next or previous question. You can check your answers immediately or when you’ve finished the quiz.