That as a relative pronoun
That is a relative pronoun. The relative pronoun that can refer to both people and things.
This is the girl that I was talking about. OR This is the girl whom I was talking about. (Here the relative pronoun that refers to the noun girl.)
Is that your car that is parked outside? OR Is that your car which is parked outside? (Here the relative pronoun that refers to the noun car.)
These are the keys that you were looking for. OR These are the keys which you were looking for. (Here the relative pronoun that refers to the noun keys.)
This is the coffee machine that I bought for my mother. OR This is the coffee machine which I bought for my mother.
Have you got something that will remove blood stains from the carpet?
Identifying relative clauses
The relative pronoun that can only be used in identifying relative clauses.
An identifying relative clause identify the noun. It tells us which person or thing we are talking about. Identifying relative clauses follow immediately after the nouns that they modify. We do not separate them by commas. Note that identifying relative clauses cannot be removed from the sentence. If we removed the clause, the sentence would make no sense or have a different meaning.
This is the house that I inherited from my parents.
Here the identifying relative clause that I inherited from my parents explains which house we are talking about - the one I inherited from my parents.
If we remove that relative clause, the sentence would read 'This is the house'. As you can see, this sentence does not make complete sentence.
Now read the sentence given below.
This is Rashmi who works with my sister.
Here the relative clause 'who works with my sister' does not identify Rashmi. It is a non-identifying relative clause. It merely adds additional information and the sentence 'This is Rashmi' makes complete sense.
You can learn more about identifying and non-identifying relative clauses here.
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