When to use ‘which’ and ‘that’

By Sharon Lapkin

Do you get frustrated trying to work out whether to use ‘which’ or ‘that’? Don’t worry if you do because a lot of people find it confusing. 

Read on and I’ll explain it simply and clearly. I’ll also provide you with three good tips to make sure you get it right.

Which and that can both function as relative pronouns, and it’s those usages that we’re paying particular attention to.

Let’s unpack it further and examine exactly what a clause is and what a sentence is.

There are four types of clauses – but there are two things they all have in common. They all contain a subject and a verb. A subject is the person or thing being described or doing the action. A verb is a ‘doing word’ that expresses a physical action, a state of being or a mental action.

 A sentence is a group of words that has a complete meaning within itself. It typically contains a subject and a predicate (the part of the sentence that contains the verb saying something about the subject), and conveys a statement, command, exclamation or exclamation. A sentence contains a main clause and one or more subordinate clauses.

When it comes to making a decision about which or that, there are two things to consider. You need to work out whether the relative pronoun – which or that – is introducing a non-essential clause or an essential clause. 

An essential clause is one that is essential to the meaning of the sentence. If we take the essential clause out the sentence it will be affected. In fact, the sentence won’t make sense without it.

For example: The passenger boarded the bus that caught on fire.

In the sentence above ‘that’ is introducing the essential relative clause. It contains essential information about the noun that precedes it. So if we remove ‘that caught on fire’, the sentence won’t make sense. This is how we know to use ‘that’ and not ‘which’.

Now let’s look at non-essential relative clauses and the role of ‘which’.

Non-essential relative clauses contain added information that can be left out without affecting the meaning of the sentence. 

For example: The garden was full of pastel-coloured roses, which were ready for picking.

If we leave out ‘were ready for picking’, the sentence still makes sense. It might not contain as much information, but it will still function as a sentence. 

Sometimes the non-essential clause is in the middle of a sentence, not at the end of it.

For example: I went to see A Star is Born, which starred Lady Gaga, and I thought it was great.

In this sentence ‘starred Lady Gaga’ is the non-essential relative clause. It can easily be left out of the sentence without affecting the meaning. Sure it leaves out some interesting information – but it still functions as a sentence.

This is how we know it is a non-essential relative clause and it should be introduced with ‘which’ not ‘that’.

Before we finish there is one vital detail that must be considered when using ‘which’ for non-relative clauses in sentences.

A comma must always precede ‘which’ unless it is preceded by a preposition (in, to, on, after, for, with). 

A sentence with a non-essential relative clause with have either two commas framing it or one and a full stop, as I have shown in the two examples.

If you are trying to work out whether to use which or that try inserting a comma before ‘which’ and if it doesn’t make sense you know to use ‘that’ instead.

These three tips will help you decide whether to use which or that:

If the relative clause introduced by ‘that’ is removed does the sentence make sense? If it does then replace ‘that’ with ‘which’.

If the relative clause introduced by ‘which’ is removed does the sentence make sense? If it doesn’t then replace ‘which’ with ‘that’?

If a comma is inserted before ‘which’ (providing it isn’t preceded by a preposition) does the relative clause make sense? If it doesn’t then replace ‘which’ with ‘that’.

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