Do you remember WH-questions words? (where, when, what, who, whose, which)
Where is your book? Where = a question word
I don't know where my book is. where = a relative pronoun
That is not a question word but it acts as a relative pronoun in a sentence.
I borrowed the book that we have to read at school.
That = relative adverb
Examples
1. PEOPLE (use who or that)
Bryson is a famous writer who/that grew up in the USA.
2. THINGS (use which or that)
It's the food which / that I can't eat.
3. POSSESSIONS / BELONGINGS (use whose)
He goes shopping with his wife, whose choices are always healthy.
4. PLACES ( use where)
This is the town where I was born.
5. TIME (use when)
This was the winter when it almost didn't snow.
6. VERBS (use what)
I don't know what he said.
