Adjective/Relative Clauses

Explanation: Adjective/Relative clauses are a group of words starting with a pronoun (that, which, who, whom, where, when, why, whose) that describes a noun.

Let's look at these two sentences:
Honolulu has a vibrant tourist driven economy. Honolulu was my hometown.

The first sentence has the main idea that we want to express. The second sentence gives extra information about the subject, Honolulu.

We can include that extra sentence in the first sentence:
Honolulu, which was my hometown, has a vibrant tourist driven economy.

Other relative pronouns:
A man who was seen earlier near the café…
A boy whom Sara ran into yesterday…
a country that has only twelve people...
A house where there was a murder…
The man whose broken-down car sits in the driveway...

Which or That?

Explanation: In formal writing, it is more common to use the pronoun which with a comma and that without a comma. That indicates that the information in the adjective clause is essential, while which indicates is just extra information.

Let's look at these two sentences:
Honolulu, which was my hometown, has a vibrant tourist driven economy.
He lives in one of only two states that hold elections from 12 a.m. on election day.

The first sentence has a clause that gives extra information about Honolulu. If you remove it, the sentence still has meaning.
The second sentence has a clause that tells us what states we are talking about. If you remove it, the sentence loses meaning.

We only do this with things.

Remember: Use commas around extra information and no commas when the information is necessary to understand the meaning of the noun and sentence.
Commas/extra: I ran into Sara, who still has not paid me back yet, in that new mall in Norwalk yesterday.
No commas/essential: This morning, I ran into a couple who just moved into that new house two doors down from us.