Thank you very much for the present you sent. (Here, "you sent" is a defining relative clause, so we don't need a comma.)
This is the story of a man whose wife suddenly loses her memory. (Here, "whose wife suddenly loses her memory" is a defining relative clause.)
He wanted to come at 2 a.m., which didn't suit me at all. (Here, "which didn't suit me at all" is a non-defining relative clause, so we need commas to set it off.)
The firm I work for is sending me to York. (Here, "I work for" is a defining relative clause.)
She climbed to the top of the monument to see the wonderful view she had been told about. (Here, "she had been told about" is a defining relative clause.)
I saw several houses, most of which were quite suitable. (Here, "most of which were quite suitable" is a non-defining relative clause.)
He expected me to pay two pounds for twelve eggs, four of which were broken. (Here, "four of which were broken" is a non-defining relative clause.)
He said the men were thieves, which turned out to be true. (Here, "which turned out to be true" is a non-defining relative clause.)
A man who answered the phone said Tom was out. (Here, "who answered the phone" is a defining relative clause.)
Dr. Field, who lectured at Browning Hall last night, predicted another earthquake. (Here, "who lectured at Browning Hall last night" is a non-defining relative clause.)
George Bernard Shaw, an Irish playwright whose photo smiles impishly at me from the wall of my study, was a famous writer. (Here, "whose photo smiles impishly at me from the wall of my study" is a non-defining relative clause.)
Tell me the reason why you didn't come to my birthday party last night. (Here, "why you didn't come" is a defining relative clause.)
He introduced me to his students, most of whom were from abroad. (Here, "most of whom were from abroad" is a non-defining relative clause.)
The car crashed into a queue of people, four of whom were killed. (Here, "four of whom were killed" is a non-defining relative clause.)
Did you find the key you lost? (Here, "you lost" is a defining relative clause.)
Jill sat on a chair, one leg of which was nearly broken down. (Here, "one leg of which was nearly broken down" is a non-defining relative clause.)
Tom came to the party in patched jeans, which surprised the other guests. (Here, "which surprised the other guests" is a non-defining relative clause.)
Most of the other guests, who were wearing evening dresses, were surprised by Tom's appearance. (Here, "who were wearing evening dresses" is a non-defining relative clause.)
I was waiting for a man who didn't turn up. (Here, "who didn't turn up" is a defining relative clause.)