先行詞が人の場合who, whose, whomが関係代名詞となりますが人以外の場合は which か that になります。
1. Restrictive examples: Relative pronoun combines two clauses and clearly identifies the noun/pronoun/noun phrase preceding the dependent clause. The dependent clause is needed to complete the meaning of the sentence. In general, American English avoids the used of ‘which’ for restrictive use, but in British English ‘which’ is freely used in both restrictive and non-restrictive cases.
The letter is from my mother.
It came this morning.
The letter which came this morning is from my mother.
The man had a dog.
It barks loudly at everybody.
The man had a dog which barks loudly at everybody.
The tree had to be cut down because of the new zoning ordinance.
It was over two hundred years old.
The tree which was over two hundred years old had to be cut down because of the new zoning ordinance.
The house was built five years ago.
I am going to buy it.
The house which I am going to buy was built five years ago.
The magazine is very interesting.
You lent me the magazine last week.
The magazine which you lent me last week is very interesting.
The building is the new museum.
You can see it up on the hill over there.
The building which you can see up on the hill over there is the new museum.
2. Non-restrictive examples: Relative clause is set off by commas and supplies extra information. The which clause can be taken out without altering the meaning of the sentence.
My neighbor’s house was completely destroyed by a fire.
It was three stories tall.
My neighbor’s house, which was three stories tall, was completely destroyed by a fire.
The story was on the front page of the newspaper.
I find the story difficult to believe.
The story, which I find difficult to believe, was on the front page of the newspaper.
My cat had hidden in the cardboard box next to the house all night.
The cat is three years old.
My cat, which is three years old, had hidden in the cardboard box next to the house all night.
The credit card you are looking for is in my wallet.
The wallet is made of leather and was expensive.
The credit card you are looking for is in my wallet, which is made of leather and was expensive.
I had to fix my car.
I bought it less three months ago.
I had to fix my car, which I bought less than three months ago.
After driving eight hours we finally arrived at Minneapolis.
It is the largest city in Minnesota.
After driving eight hours we finally arrived at Minneapolis, which is the largest city in Minnesota.