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.

Who or whom?

The use of who and whom often causes confusion, so it's a topic worthy of discussion.

Let's begin by looking at the two sentences below:

(1) The man _____ hit you is a bouncer.
(2) The man _____ you hit is a bouncer.

Which of these requires who and which requires whom?

To answer that question, we need to first introduce two definitions:

who is a relative pronoun functioning as subject
whom is a relative pronoun functioning as object

(In grammar, a subject is typically the doer of the action and an object the recipient of the action. So in The dog bit Alice, The dog is the subject of the sentence and Alice is the object.)

Next, we ask the question: Who did the hitting?

The answer to that question will help us find the subject of the verb hit.

In (1), the man did the hitting. We can see this more clearly by simplifying (1) to (1.1):

(1.1) The man hit you; the man is a bouncer

So the man is the (underlying) subject of hit in sentence (1). Consequently, we use who because who is subjective:

(1.2) The man who hit you is a bouncer.

In (2), you did the hitting. Again, it helps to simplify sentence (2) to (2.1):

(2.1) You hit the man; the man is a bouncer.

Sentence (2.1) shows that the man is the (underlying) object of hit. Consequently, we use whom for (2) because whom is objective:

(2.2) The man whom you hit is a bouncer.

The discussion above notwithstanding, the use of whom is considered rather formal. In spoken or informal English, who is preferred over whom in object position. But when there is a preceding preposition, whom is the only choice:

(3) The girl to whom he spoke is Angela.
(4) With whom did you have dinner?