This is the 55th most frequent Estonian word.
mille
The Estonian word "mille" means "of what" or "whose" in English.
Interrogative pronoun 'mille' in combination with 'pärast' asks about the cause or reason.
Kas tead, mille pärast ta hilines?
Do you know why he was late?
Relative pronoun 'mille' refers to 'book', connecting the dependent clause to the noun.
See on raamat, mille ma eile lugesin.
This is the book that I read yesterday.
Interrogative pronoun 'mille' asks about the purpose or reason for an action.