Relative clauses in English (i.e. those which modify a preceding noun, noun phrase or pronoun) are introduced by a relative pronoun – either ‘that’ or ‘which’, depending on the circumstances.
Where the relative clause adds further information about the noun or pronoun (a so-called non-restrictive clause), ‘which’ must be used: e.g. ‘The winning car in the rally, which was made in Germany, was a Porsche’; or ‘The year’s biggest seller, which outsold its nearest rival by two to one, was made by Sony’.
In these two examples, the car or the item being referred to can be identified without the relative clause.
Where the clause identifies the noun or pronoun to which it refers, ‘that’ would generally be used: ‘The winning car was the one that was made in Germany’ or ‘It was the Sony that became the year’s biggest seller’. One could rewrite these sentences to omit the relative pronouns, but if a relative pronoun is used, then because the clause supplies information necessary for the subject to be identified, ‘that’ is the preferred option.
However, ‘which’ may also be used in identifying restrictive clauses, in particular:
- to give emphasis: e.g. He bought the book which would change his life in a second hand shop.
- to avoid repetition of ‘that’: e.g. I knew that I would eat the chocolates which I had bought without sharing them with my wife.
- after a preposition: e.g. He couldn’t wait to see the film in which his favourite actor was starring.
