ASK DOCTOR DOROTHY PASTENSE FULLSTOP:
Are 'that' and 'which' interchangable?
For a while this particular example was scheduled to go into the What's the difference section of the website, but something changed... it became a full-blown question which provokes arguments in some areas.
Okay, let's take it from the beginning.
That and which are examples of relative pronouns. And what we mean by interchangeable is using that instead of which or which instead of that in sentences and clauses.
First, there are examples where you can't change them around. Clearly in these examples the meaning is obvious: Which pen is mine? That pen is mine! You can't interchange those otherwise the meanings will be different. In addition, which, not that, is used with plural nouns, so you can say which pens but not that pens. Nobody has a problem with those, but this is not the area of debate.
The problem is with relative clauses.
From Not4GrammarBores: This is a clause within a sentence that is introduced by pronouns such as who, which, that, whose, where, etc. For example: A radio is a device that plays music. A waiter is a person who serves food in a restaurant. A university is a place where people study subjects. My friend, who is a dentist, works in Radom. Sometimes these pronouns are left out, particularly when speaking, when the relative pronoun is the subject of the verb: The museum [that] I wanted to visit is closed today. Are these the papers [that] you were searching for?
There are also two types: defining relative clauses and non-defining relative clauses.
Look at this example: The example, which was added to the A-Z extras section of TEE, is full of mistakes. The words between the commas form a relative clause. If you removed these words, the sentence would still make grammatical sense; the clause simply adds additional information and so is called a non-restrictive relative clause.
Now look at this example: The example that was added to the A-Z extras section of TEE is full of mistakes. The words 'that was added to the A-Z extras section of TEE' form a relative clause, but here it tells us specifically that the specific example added to the A-Z section of TEE was full of mistakes. There are no commas added (which are added to non-restrictive relative clauses), so this is called a restrictive relative clause; the sentence is complete and does not contain additional information, usually added between brackets.
So that's established that, now what's the problem?
Well, often the use of which/that are interchanged in non-restrictive relative clauses, and realistically this isn't always a problem. We all pretty much do it and it's something that is not an issue when teaching English.
But not everyone agrees, and unfortunately it is those in the media and in high-end jobs who believe that there are fixed rules about them, even though there is no substantiation for it.
What they say is this: You can say The example, which was added to the A-Z extras section of TEE, is full of mistakes, but you can't say The example, that was added to the A-Z extras section of TEE, is full of mistakes. They believe that you cannot use that in a non-restrictive clause. Likewise, you can say The example that was added to the A-Z extras section of TEE is full of mistakes but not The example which was added to the A-Z extras section of TEE is full of mistakes as this is a restrictive clause and you can't use which instead of that.
Only one of those examples is correct. For non-restrictive relative clauses (that's those with the commas), it's true that which, not that, should be used. However, for those with restrictive relative clauses (those without the commas), either which or that can be used. So it's okay to say the door which is next to the window is open or the door that is next to the window is open, or She chose the book which was the most interesting or she chose the book that was the most interesting.
So where has this 'rule' come from? Well, back in the 19th century various 'experts' that called themselves grammarians tried to fix rules that they thought would be a good idea. Indeed, there is an example where a grammar book back in the 18th century called The English grammar of William Cobbett (by Cobbett, William, 1763-1835) was 'corrected' in the 19th century for this rule, and even Henry Fowler, a grammarian whose book A Dictionary of Modern English Usage in the early 20th century, only advises that it would be good to follow such rules, but does not say that such rules exist. Writers in the past have not stuck to these 'rules'.
Another source could well be American English where many of its authorities and style guides recommend that you should use that and not which to introduce a restrictive relative clause, so you should say she ate the cake that looked delicious and not she ate the cake which looked delicious. However, this is only a recommendation; it is not a rule. If we were to summarise this whole issue, that is often considered to be used more in conversation and which in more formal contexts.