It is really confusing where to use that or which*. Some English speakers,especially the non-native, tend to use them without a definitive rule in mind;whichever one comes onto their tongue at the point of speaking,they use. But as stated in one of my earlier posts, things like that are only accepted to some extent in informal English. In a formal English, one has to know when exactly to use which or that. Let’s now get to business with the rules involved.
BASIC RULE:That and which are relative pronouns ( aside who and whom) used to introduce relative/adjectival clauses qualifying nouns in sentences. Use that or which for things. For persons, use who or whom.
1. The man who lost his wife in death is very depressed now. [ The relative clause here is italicized. This clause is actually qualifying the noun, man and that noun is a person; hence, who is used, not which or that]
2. The pen, which I bought, is lost.[ not who; the relative clause is qualifying a thing]
Having considered the basic rule, let’s look at the difference between that and which because although they are both used for things, there is a difference in their usage. Let’s analyse the following sentences.
1. My pen, which has a red cover, is lost.
This sentence means I have only one pen.And for the sake of identification, I added the relative clause, which has a red cover. I can actually take it off, but the sentence will still retain its meaning: My pen is lost[ I have a pen and it is lost]. My pen, which has a red cover, is lost.[ I have a pen, and it has a red cover. This pen is lost. It is just a pen we are referring to].
This kind of clause that can be removed from a sentence without altering its meaning is called a nonessential clause, and you use which to introduce it just as I have done here. Take note of the comma(,) before and after them.
2. My pen that has a red cover is lost.
Here, I have more than a pen, and one of them- having a red cover- is lost. We cannot take off the relative clause in this sentence; it is an essential clause. When we do, the meaning of the sentence will be altered.
As I already stated , this sentence means I have more than a pen, and one of them is lost. Removing the relative clause, the sentence becomes: My pen is lost, which means I have a pen and it is lost. This is in sharp contrast to what I have just explained.
If you mean to describe the only thing/group there is, ‘which’ is the better option of the two as discussed in sentence 1. But if there is a lot and you want to single out one/a group by description, use that.
1. use which when you mean to just add a description that is not necessary. In other words, use which to introduce nonessential clauses.
2. use that when the description is necessary to single out one from a lot. We use that to introduce essential clauses.
1. The car, which my dad bought recently, has started developing faults. [The car has started developing faults. Referring to one car, not one car out of a lot, so which my dad bought recently is not really necessary; it is only added for description’s sake. We therefore call it a nonessential clause, and ‘which’ introduces such]
2. The house that is painted yellow needs renovation. [ We have other houses, but the one painted yellow needs a renovation. That is painted yellow is an essential clause and no doubt ‘that’ is used, not which]
I know this can be kind of confusing. Let’s test ourselves with the following questions.
1. The cars (that/which) are painted blue belong to me.
2. The answers, ________ you can find in the back of the book, are sometimes incorrect.( which/that)
3. Walden Pond, ______ was written in the mid-1800s, remains a popular book among romantic and individualistic Americans.( which/that).
4. She prefers to watch movies ______ make her cry.( which/that)
5. He bought all the books ________ are required for the course.( which/that)
1. that: there are other cars, but I am singling some out of the lot. Without the that relative clause( essential clause), the meaning of the sentence will be altered.
2. which: referring to only one group, answers, not some answers out of the lot. So, the which relative clause( nonessential clause) is not really necessary because there are no other answers anywhere according to the sentence. Removing it, therefore, does not alter the meaning of the sentence.
3. which: only one Waldon Pound is being referred to, so ‘which‘ is the right pronoun to use and introduce the relative clause. If there were some other Waldon Pond, that will be used to show the distinction. But since it’s just one, which is used, making that clause a nonessential one.
4.that: there are other movies, but she prefers a particular kind. That is therefore used to introduce the essential clause, separating those movies from the lot.
TAKE NOTE: When that, this, or those is used to introduce the first part of the sentence, use which in the relative clause whether it is essential or not.
1. This is the book which I bought.
2. Those books, which I used to read, are now very boring to me.
Another way is to remove ‘which’ completely. So the above sentences become: 1. This is the book I bought . 2. Those books I bought are now very boring to me.
*That and which have quite a number of uses, but in this studies, our focus is on them as relative pronouns.