Understanding the Difference Between Which and That

Few grammar points cause as much confusion as the choice between 'which' and 'that'. Both words introduce clauses that give additional information about a noun, but they are not interchangeable. Using the wrong one changes the meaning of a sentence in ways that can confuse your reader without either of you realising it. Once you understand the underlying logic, the distinction becomes quite intuitive.

Restrictive Clauses: Use 'That'

A restrictive clause narrows down or identifies which specific noun you are referring to. Without this clause, the sentence would mean something different or become unclear. In these cases, you use 'that'. There is no comma before 'that' in a restrictive clause.

Consider this example: 'The book that I borrowed from the library is overdue.' The clause 'that I borrowed from the library' is essential. It identifies which specific book is overdue, distinguishing it from all other books you might own or have access to. Remove it and you are left with 'The book is overdue,' which tells your reader nothing about which book you mean.

Non-Restrictive Clauses: Use 'Which'

A non-restrictive clause adds extra information about a noun that has already been clearly identified. The clause is interesting or useful, but removing it would not change the core meaning of the sentence. You use 'which' here, and you always place a comma before it.

Compare this example: 'My favourite book, which I read every summer, is falling apart.' The noun 'my favourite book' is already specific. The clause 'which I read every summer' simply adds a charming detail. You could remove it entirely and the sentence would still make complete sense: 'My favourite book is falling apart.'

The Comma Test

A simple way to decide which word to use is the comma test. If your clause needs a comma before it, use 'which'. If there is no comma, use 'that'. The comma signals that the clause is non-restrictive and therefore optional. No comma signals that the clause is restrictive and essential.

Why the Difference Matters

Consider these two sentences side by side. 'The cats that sleep on the roof belong to my neighbour.' This implies there are multiple cats, and only the specific ones that sleep on the roof belong to your neighbour. 'The cats, which sleep on the roof, belong to my neighbour.' This implies all the cats (you only have one group in mind) belong to your neighbour, and the detail about the roof is simply additional context. The meaning is noticeably different.

A Note on British and American English

It is worth noting that in American English, grammarians apply this rule quite strictly. In British English, 'which' is sometimes used where an American would insist on 'that', even in restrictive clauses. However, for formal writing, professional communication, and IELTS preparation, following the distinction described above is always the safer and more universally respected choice. Learning it correctly from the start will serve you well across all formal contexts.

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