That vs which

Can you use that whenever you use which? If we look at the pair of sentences below, the answer would appear to be yes:

(1) The deer which was shot miraculously survived.
(2) The deer that was shot miraculously survived.

Sentences (1) and (2) show that both which and that can be used without any change in meaning.

However, consider the next pair of sentences below:

(3) The deer, which was shot, miraculously survived.
(4) *The deer, that was shot, miraculously survived.

Sentences (3) and (4) are not that different from the corresponding (1) and (2). However, while both sentences (1) and (2) are acceptable, only sentence (3) is acceptable but not sentence (4), as indicated by the asterisk (*).

Why is that so? What difference do the commas make?

The answer lies in the difference between restrictive and non-restrictive clauses. 

A restrictive clause limits or restricts the meaning of the noun by introducing information that is essential for understanding the sentence. It is not set off by commas.

Sentences (1) and (2) above show the use of restrictive relative clauses.

The restrictive relative clause in each limits the meaning of the noun deer so that the reader knows which deer was being referred to.

If we remove the restrictive relative clause in (1) and (2), the resultant sentence would mean completely different from the ones with the restrictive clause.

Specifically, sentence (5) below does not have the same meaning intended by sentences (1) and (2):

(5) The deer miraculously survived.

Which deer miraculously survived? The one that injured itself? The one attacked by a tiger? That piece of information is missing in sentence (5).

A non-restrictive clause adds information that could be useful but is not essential for understanding the sentence. It is set off by commas.

Sentence (3) above shows the use of a non-restrictive relative clause. The pair of commas renders the relative clause which was shot non-restrictive. The non-restrictive relative clause merely adds the information that the deer was shot and leaves open its interpretation in the sentence.

If we remove the non-restrictive relative clause in (3), the meaning of the resultant sentence will not be very different from the one intended by the original sentence, since the information that the deer was shot is peripheral to understanding the sentence.

So, back to the question we posed at the beginning. Sentence (4) is unacceptable because the relative clause is used non-restrictively, in which case only which is permitted. When a relative clause is used restrictively, the relative pronouns which and that are interchangeable. The choice of one over the other is a matter of style, with some writers preferring to use only that for restrictive clauses.