"That" or "Which"

This is one of the most common mistakes out there, and understandably so. “That” is a restrictive pronoun. It’s vital to the noun to which it’s referring.  e.g., I don’t trust fruits and vegetables that aren’t organic. Here, I’m referring to all non-organic fruits or vegetables. In other words, I only trust fruits and vegetables that are organic. “Which” introduces a relative clause. It allows qualifiers that may not be essential. e.g., I recommend you eat only organic fruits and vegetables, which are available in area grocery stores. In this case, you don’t have to go to a specific grocery store to obtain organic fruits and vegetables. “Which” qualifies, “that” restricts. “Which” is more ambiguous however, and by virtue of its meaning is flexible enough to be used in many restrictive clauses. e.g., The house, which is burning, is mine. e.g., The house that is burning is mine.

"that" effectively makes the right hand clause into an adjective. For example "This is the house that Jack built" -- here "that Jack built" is part of the description of the house.

"which" (or more precisely, ", which", since you need the preceding comma) just adds a descriptive phrase which supplements the sentence. For example "This is Jack's house, which is quite large".

The essential difference is that "This is the house" does not narrow down WHICH house you're referencing, so you add "that Jack built" to make that distinction. On the other hand "This is Jack's house" DOES narrow down which house you're referencing, and so you don't need to narrow it down further, and so you don't need "that".

When proofreading a peer’s article on the solar system, I realized that she, and I, are unsure of the proper use of “that” and “which” in a sentence. Below is [SIC] two examples of the same sentence, one using “that” and the other “which.”

• “To our knowledge, it is the only body in the solar system which currently sustains life, although several other bodies are under investigation.”
• “To our knowledge, it is the only body in the solar system that currently sustains life, although several other bodies are under investigation.”

Which is the correct sentence, and what is the general rule of thumb?
Justin, I’ll give you the answer now, rather than making you read to the end of the whole article: the second version of that sentence, using that is correct.

When To Use “That” and When To Use “Which”

Before I come on to the “that”/”which” rule, just a reminder that “who” should always be used when referring to people.

• The boy who threw the ball.
• This is the woman who always wears a black shawl.

When referring to objects, though, the rule for using “that” and “which” correctly is simple:

• THAT should be used to introduce a restrictive clause.
• WHICH should be used to introduce a non-restrictive or parenthetical clause.

If that leaves you more confused than when you began this article, read on…
A restrictive clause is one which is essential to the meaning of a sentence – if it’s removed, the meaning of the sentence will change. For example:

• Chairs that don’t have cushions are uncomfortable to sit on.
• Card games that involve betting money should not be played in school.
• To our knowledge, it is the only body in the solar system that currently sustains life…

A non-restrictive clause can be left out without changing the meaning of a sentence. Non-restrictive clauses are either in brackets or have a comma before and after them (or only before them if they come at the end of a sentence):

• Chairs, which are found in many places of work, are often uncomfortable to sit on.
• I sat on an uncomfortable chair, which was in my office.

Why You Need to Use “That” or “Which” Correctly

Changing that to which or vice versa can completely change the meaning of a sentence. Consider the following examples:

• My car that is blue goes very fast.
• My car, which is blue, goes very fast.

The first sentence uses that – suggesting I own more than one car (and even implying my other cars might not be so fast). This is what happens if we leave out the clause and write:

• My car that is blue goes very fast.
• My car goes very fast.

The sentence’s meaning has changed: the reader does not know which one of my cars goes very fast.
However, the sentence using which simple informs the reader that my car is blue. We can take the clause out without losing any essential information:

• My car, which is blue, goes very fast.
• My car goes very fast.

“That” and “Which” in Common Usage

It is common today for which to be used with both non-restrictive and restrictive clauses, especially in informal contexts:

• Who ate the cake that I bought this morning?
• Who ate the cake which I bought this morning?

The clause “that I bought this morning” is essential to the meaning – I’m not asking about a cake which I bought yesterday, or this afternoon. Therefore, the first example using “that” is the correct one, but many people would not consider the second ungrammatical.
It is, however, incorrect even in informal contexts to use that for a non-restrictive or parenthical clause. For example, these sentences would be considered incorrect:

• This computer, that I have never liked, is very slow.
• The blue desk, that my father gave me.

An easy way to watch out for these is to look for instances where you have a comma followed by the word that. If I’d know this years ago, it would have saved me a lot of frustration with Microsoft Word!
Even though the usage of which has been relaxed to some extent, it is still better to keep your writing as clear as possible by using which for only non-restrictive clauses, and that for restrictive ones.
So, to return to Justin’s example:

• “To our knowledge, it is the only body in the solar system which currently sustains life, although several other bodies are under investigation.”
• “To our knowledge, it is the only body in the solar system that currently sustains life, although several other bodies are under investigation.”

The second sentence, using that is correct, but many people would consider the first sentence permissible too. In a formal context such as a scientific paper, it is better to use that for total clarity.

In certain phrases in English, you can use “which” or “that” such as, “Monday is the day that/which I hate the most!” However, sometimes you can only use “which”. For example: “I’m from Moscow, which is the capital of Russia.”

The good news is that you can always use “which”, although most native English speakers prefer to use “that” whenever it’s possible. In this blog, we’re going to examine the grammar rules that govern the use of these annoying little words that cause so much confusion for students.
From First Certificate in English level onwards, you will be expected to be able to understand and use these words correctly and may be graded on your use of them in both the Writing and Use of English papers.
To clarify, we are not talking about the use of “which” as a question word (e.g. Which is bigger; Italy or France?) but as a linking word in subordinate clauses.

Subordinate clauses

A subordinate clause is a clause which functions as part of another clause (e.g. as subject, object or adverbial in the main clause of a sentence). Simplified, a subordinate clause is part of a bigger sentence and exists to define the subject or object, or to provide extra information. Look at the following examples:

1. Subordinate clause defining subject:

“The city that/which I love the most is Barcelona.”

In this example, the subordinate clause is underlined and it forms part of the subject. You can use “that” or “which” in this sentence. Imagine the sentence without the clause: “The city is Barcelona”. This sentence is grammatically correct but lacks meaning. It is ambiguous. Therefore, the subject of the sentence should be “The city that I love the most…”. In this case, the subject is defined and the sentence has meaning.

2. Subordinate clause defining object:

“I love languages that/which are easy to learn.”

In this example, the subordinate clause underlined forms part of the object. Without the clause, the sentence is just “I love languages” which isn’t quite what the speaker is trying to say. Again, in this example both “that” and “which” are possible.

3. Subordinate clause providing extra, non-essential information:

“My car, which is 10 years old, needs to be repaired regularly.”

In this example, the subordinate clause is providing extra information, which is neither defining the subject or the object, nor is the information essential to the meaning of the sentence. Imagine the sentence without the subordinate clause: “My car needs to be repaired regularly.” The meaning of the sentence here is the same. The subject is clearly defined. True, the sentence has a little less information but it doesn’t affect the meaning. In this example, only “which” is possible.

Also, notice the comma separating the subordinate clause from the subject. Although the subordinate clause here relates to the subject, it is not part of it. Therefore we use a comma to show this separation.
These three examples are essentially the three types of subordinate clause. Numbers 1 and 2 are examples of what are known as “defining clauses” while number 3 is an example of a “non-defining clause”.

Conclusion

1. Remember, clauses that define the subject or the object can be introduced with “which” or “that”. It is useful to know that most native English speakers prefer to use “that” in these cases.
2. Clauses that simply provide extra information can only be introduced with “which”. It is useful to recognise that such clauses are more common in written English than in spoken English.
3. When using a non defining relative clause with “which”, you need to use a comma to separate the clause from the subject or object.

N.B. “Which” and “that” are not the only words that introduce subordinate clauses. There are also “who”, “where”, “when”, “whose”. However, “which” and “that” are the most common relative pronouns used to define or give extra information about things, while the others are used for people, places, time and possession respectively.