Difference Between 'That' and 'Which'

While interviewing for an English language teaching position, the interviewer asked a candidate to explain the difference between ‘that’ and ‘which’. The interviewee replied, “Both ‘that’ and ‘which’ are pronouns and they are interchangeable.” She justified her answer with these examples:

I have sold the car that I bought last year.

I have sold the car which I bought last year.

Unconvinced, the interviewer said, “The words ‘that’ and ‘which’ are not always interchangeable.” Then he asked the candidate to explain the difference between a restrictive and a nonrestrictive clause. As the interviewee was not familiar with the nomenclature, the interviewer asked her to write these two sentences on the board and explain the difference in meaning between the sentences:

The car that I bought last year is not in good condition.

The car, which I bought last year, is not in good condition.

These two sentences have different meanings.  The first sentence implies that I have more than one car and the car that is not in good condition is the one I bought last year. The clause ‘that I bought last year’ is essential to the meaning of the sentence and as it restricts the car, it is called a restrictive clause (or an essential clause). The restrictive clause in a sentence identifies the word it modifies and the clause is not separated by commas.

The second sentence implies that I have only one car and an additional piece of information about the car is that it was bought last year. Even if the clause 'which I bought last year' is removed, the meaning of the sentence does not change. It is nonrestrictive and nonessential. A nonrestrictive clause is a clause that gives additional information in the sentence and it is always separated by commas.

Now the question is when to use ‘that’ and ‘which’. The rule is that if a sentence needs the clause use ‘that’ and if it does not need the clause use ‘which’. Here is one more example: 

The computer that is in the bedroom was presented by my aunt.  (I have more than one computer.)

The computer, which is in the bedroom, was presented by my aunt. (I have only one computer. That it is in the bedroom is additional information)

Here is another example of restrictive and nonrestrictive clauses with ‘who’:

My brother who is a dermatologist is in the US.

This sentence implies that I have more than one brother because the clause 'who is a dermatologist' is restrictive as it describes and identifies the brother.  It is essential to the meaning of the sentence.

My brother, who is a dermatologist, is in the US. 

This sentence implies that I have only one brother. That he is a dermatologist is additional information. The nonrestrictive clause ‘who is a dermatologist’ is separated by commas in the sentence. Even if the clause is removed the meaning of the sentence does not change.

Does a comma matter? Yes, it does. Look at these two sentences:

Let’s eat, aunt.

Let’s eat aunt.

Without the comma the meaning changes completely!