To explain this, I need to describe the difference between restrictive clauses and non-restrictive clauses. If you’ve never heard of these, please don’t panic.
Restrictive and non-restrictive clauses do what they say they do.
A restrictive clause restricts the noun. You can’t get rid of it. It gives vital information about what you’re talking about.
Restrictive clauses start with ‘that’. The word ‘that’, for those of you who like to name things, is a defining or restrictive pronoun.
Think about the following:
Staff that are designated First Aid Officers must attend the training.
If we take away the words ‘that are designated First Aid Officers’ we are left with all staff. The sentence has a completely different meaning. The sentence is no longer true. It becomes:
Staff must attend the training.
Here’s another example:
Dogs that bark at night drive me crazy.
What happens if I take away ‘that bark at night’? I’m left with a sentence which just isn’t true. I’m left with:
Dogs drive me crazy.
Non-restrictive clauses provide additional information about something, but can be left out without changing the sentence’s meaning.
The memo, which is on my desk, includes all the details of the training.
The non-restrictive clause gives you additional information about the memo, but it doesn’t define it. If I take it away the sentence is still true.
The memo includes all the details of the training.
What happens if we use ‘that’ in this sentence?
The memo that is on my desk includes all the details of the training.
This new sentence indicates exactly which memo I am referring to—it defines it.
Here’s another example.
The dog, which was barking, was in the backyard.
In this sentence, ‘which was barking’ adds a fact about the only dog we are talking about. If I take it away, I still have a true sentence.
The dog was in the backyard.
Try it again with ‘that’.
The dog that was in the backyard was barking.
This sentence tells you which dog was barking. It defines it.
If you’ve looked closely at the examples sentences you will have noticed that when we use ‘which’ we use commas. We don’t use commas with ‘that’.
Use ‘that’ when you can’t get rid of the information and ‘which’ when you can.