Guide to English grammar

To who was the parcel addressed?
On what evidence was it decided to make the arrest?

For who and whom, • 26(3).

NOTE
Since comes before when even in informal English. This often expresses surprise. A question with “How long...?” is more neutral.
Since when has this area been closed to the public?

A practical guide to English grammar 27

26 Question words: more details

1 What, which and whose before a noun
These question words can be pronouns, without a noun after them.
What will be the best train?
There are lots of books here. Which do you want?
Whose was the idea?
They can also be determiners, coming before a noun.
What train will you catch? (You will catch a train.)
Which books do you want? (You want some of the books.)
Whose idea was it? (It was someone's idea.)
Which can come before one/ones or before an of-phrase.
Which ones do you want?
Which of these postcards shall we send to Angela?

2 The use of who, what and which

Who always refers to people. Which can refer to people or to something not human. What refers mostly to something not human, but it can refer to people when it comes before a noun.

Human Who is your maths (GB)/math (US) teacher? Which teacher do you have? What idiot wrote this?

Non-human Which supermarket is cheapest? What book are you reading? What do you do in the evenings?

Who is a pronoun and cannot come before a noun or before an of-phrase. (NOT Who teacher do you have? and NOT Who of the teachers do you have?)
There is a difference in meaning between what and which.