This is the letter for which I've been waiting -> ....... for.
This is the letter which I've been waiting for
This is the shop from which Sue bought her bike -> ... from
This is the shop which Sue bought her bike from
United were the best team against which we played. -> ... against
United were the best team which we played against
Add a relative pronoun into the sentence: The girl I met was a friend of Philip's
The girl WHO / WHOM I met was a friend of Philip's
Add a relative pronoun into the sentence: The book I read didn't explain the problem
The book WHICH / THAT I read didn't explain the problem
Add a relative pronoun into the sentence: The friends I met last night send you their love
The friends whom / that / who I met last night send you their love
Tim is someone to whom I hardly every write. -> ... to
Tim is someone whom I hardly every write to.
Ravenna was the most interesting town in which we have stayed. -> ... in
Ravenna was the most interesting town which we have stayed in.
Add a relative pronoun into the sentence: The meal we ate wasn't very tasty
The meal THAT / WHICH we ate wasn't very tasty
Do you know by whom this book was written? -> ... by?
Do you know whom / who this book was written by?
Add a relative pronoun into the sentence: Friday was the last time I saw Jim
Friday was the last time that I saw Jim