Relative clauses are clauses that begin with one of these relative pronouns: 

who/whom, that, which, whose, where, when 

Relative Clauses have 2 main purposes:

1) Define or Identify the Preceding Noun

We mostly use them to define or identify the noun that comes before them. Here are some examples:

• Do you know the girl who started going to our school yesterday?
• Can I have the pen that I gave you last night?
• A laptop is a computer which you can carry around in your backpack.
• I don’t want to meet at a cafe whose coffee is so bad.
• I want to stay in a place where there are many historical sites.
• Monday was the day when I learned to drive a car!

* There is also a relative pronoun whom, but we rarely use it in normal spoken English. It’s more seen in old English.It can be used as the object of the relative clause. For example: My science teacher is a person whom I like very much.

The preceding noun determines which relative pronoun you can use. “That” is the most widely used and you can often pick between who/that for a person, or which/that for a thing: 

If the preceding noun is…

A person A thing  

who/that, whose which/that, whose

• Do you know the boy who ..
• She was a woman that ..
• An orphan is a child whose parents .

– Do you have a phone which ..

– The pine is a tree that ..

– This is a magazine article whose author ..

Note on whose: The relative pronoun whose is used in place of a possessive pronoun. It must be followed by a noun. Example: There’s a boy in my class whose father is a professional soccer player. (There’s a boy in my class. His father is a professional soccer player.)

Note on where and when: The relative pronouns where and when are used with place and time nouns. Examples: Harvard is a school where students from around the world are educated. 2001 was the year when terrorists attacked the International Trade Center in New York City.

2) Giving Extra Information

Some relative clauses are used to give extra information about the preceding noun. (not to define or identify the preceding noun) For example: 

• My history professor, who came to Turkey in 1979, likes to visit historical sites.
• The heavy snow, which was common in the winter, created high water levels in the lakes after it melted in the spring.
• Einstein, who is known around the world, is famous for his theory of relativity.
• The girl, whose parents both work as teachers at the school, started a fight in the hallway.
• My father’s company, which makes tablets, is moving soon from New York to Los Angeles.
• Over winter break I’m going to visit Canada, where my sister lives.

Note 1: Relative clauses which give extra information must be separated off by commas. 

Note 2: We can’t use the relative pronoun that to introduce an extra-information (non-defining) clause about a person. Wrong: Neil Armstrong, that was born in 1930, was the first man to set foot on the moon. Correct: Neil Armstrong, who was born in 1930, was the first man to set foot on the moon.

When You Can Leave Out the Relative Pronoun

There are two common occasions, particularly in spoken English, when you can omit the relative pronoun:

1. When the pronoun is the object of the relative clause, it can be left out. In the following sentences the pronoun that can be left out is enclosed in (brackets):

• Do you know the man (who/m) he’s talking to?
• Where’s the pen (which) I gave you last night?
• I haven’t read any of the books (that) I got for my birthday last year.
• I didn’t like that girl (that) you brought to the party.
• Did you find the homework assignment (which) you lost?

Note: You cannot omit the relative pronoun a.) if it starts a non-defining relative clause, or, b.) if it is the subject of a defining relative clause. For example, who is necessary in the following sentence: What’s the name of the girl who won the tennis tournament?

Sometimes it’s helpful to break up the sentence into two smaller sentences. That way you can see the main clause and the relative clause separated to determine whether the relative pronoun is the subject (can’t omit) or the object (can omit). 

For example:

Do you know the man (who/m) he’s talking to?  

Break it up: He’s talking to a man. Do you know him?

In this case, the man is the object of the sentence, so you can leave out the relative pronoun “who.”

But now try this sentence: Do you know the girl who started going to our school yesterday?  

Break it up: A girl started going to our school yesterday. Do you know her?

In this case, the girl is the subject of the sentence, so we cannot leave out the relative pronoun “who.”

2. When the relative clause contains a present or past participle and the auxiliary verb to be. In such cases both relative pronoun and auxiliary can be left out:

Present/Past Participle is in blue.

Auxiliary verb “to be” is in green.

• Who’s that man (who is) standing by the gate? >>> Who’s that man by the gate?
• The family (that is) living in the next house comes from Slovenia.
• She was wearing a dress (which was) covered in blue flowers.
• Most of the parents (who were) invited to the conference did not come.
• Anyone (that is) caught writing on the walls will be expelled from school.