A defining relative clause is used to give information about a noun in a sentence. This information is needed by the listener or reader to understand the whole thought in the sentence.
A lawyer is a person who represents ordinary people in a legal proceeding.
The man who helped carry my stuff has just left.
We use who to provide more details about a person.
A policeman is a person who helps keep the street safe.
We use which or that to provide more details about a thing.
A ring is a thing which you wear around your finger as an accessory.
The dress that I wore last summer is beautiful.
We use where to provide more details about a place.
That’s the restaurant where we had our first date.
The city where I was born is a popular tourist destination.
Non-defining relative clauses provide key information therefore commas (,) are not needed.
If the object of the sentence is the noun which the relative clause describes (commonly found at the beginning of the sentence), the words that, which or who can be omitted.
Examples
1. The girl who is staring at me is my former girlfriend.
The girl is the subject of the sentence. You cannot omit who.
2. The woman (who) Andy has been dating for years is my cousin.
Andy is the subject of the sentence.
The woman is the object of the sentence.
When rewritten: Andy has been dating a woman for years. The woman is my cousin.
Here, Who can be omitted.
3. The house you showed me is in a quiet neighborhood.
You is the subject of the sentence.
The house is the object of the sentence.
When rewritten: You showed me a house. The house is in a quiet neighborhood.
Which / that is unnecessary.
4. The car which is on the left side is a luxury car.
The car is the subject of the sentence.
When rewritten: The car is on the left side. It’s a luxury car.
Which / that is necessary.