Which | That
The grammatical rule of when to use that and when to use which has confused both native English speakers as well as those learning the language. It is often quite difficult to know which of these two terms to use, due to the very subtle differences between the two. The first thing to know is that they are both relative clauses, meaning that they modify the sentences which they are attached to.
Who
If your relative clause is about people, then you don't need to use that or which. Instead, you should use who:
- The boy who fell over
- The woman who had eight children
Which
Which introduces non-restrictive clauses. This type of clause provides non-essential additional information about the object of the sentence which can be removed without the sentence losing its coherency:
- The report was highly informative
- The report, which was released yesterday, was highly informative.
You cannot use that to introduce a non-defining relative clause. If which is not entirely appropriate, you can also use who, whom, and whose to introduce this kind of clause:
- The man, whose wife was away, was spending most evenings in the bar
Which can also also introduce defining relative clauses, which modify the whole clause, not just the object;
- They can't come after all, which is a shame
In this case, the supplementary information shows that the writer is sad that the people were unable to come. Without this additional information, we would not know how the writer felt about this turn of events. This kind of clause can only be introduced by which.
That
That, on the other hand, can only be introduced with sentences containing defining restrictive clauses. These clauses are essential to the sentence and cannot be removed without changing the entire meaning of a sentence:
- Children's books that don't have happy endings should be banned
- The girl screamed when she saw the spider that had just run across the floor
The supplementary information provided after that (happy endings and spiders running across the floor respectively) gives us a fuller picture and meaning to the picture that is lost if the clause is removed:
- Children's books should be banned
- The girl screamed when she saw the spider
It should be noted that it is more common today to use which for both defining and non-defining relative clauses, as noted in the section on which above. However, The Whole Proof believes that it is good practice to maintain the boundaries - and thus keep your writing as clear as possible - by reserving which for non-definining relative clauses and that for defining relative clauses.