You must have struggled at least, once, with when to use that or which in your writing and eventually had to go with your guts. If it helps, you should know that you are not the only one and that the difference between that and which is one of the difficult grammatical concepts in the English language. The two words serve a similar purpose, but they are not the same. Using either of the two in a sentence gives the sentence a different meaning.
Let us break down the rules governing their use.
WHEN TO USE “THAT”
Before you decide to use that or which, you should analyse the clause it seeks to introduce to know whether it is a restrictive clause or a non-restrictive clause.
What is a restrictive clause? A restrictive clause implies that the information in the clause is critical to understanding the preceding noun. A restrictive clause should always use that.
Examples:
Leigh’s phone (noun) that has the gold band (restrictive clause) was bought at a thrift shop.
The sentence above means that Leigh has other phones; therefore, the particular phone being referred to in the sentence above needs to be distinguished (by its gold band) from her other phones.
A cat that adores its owner is usually agreeable to strangers.
This sentence implies not all cats adore their owners. So, we use that to provide the reader that important information about this particular cat.
WHEN TO USE “WHICH”
Which is used to introduce non-restrictive clauses.
What is a non-restrictive clause? A non-restrictive clause is not critical to the understanding of the noun in the sentence.
In this situation, you use which and a comma (another difference between the use of that and which). The comma is used to separate the non-restrictive clause from the rest of the sentence.
Examples:
My gold shoe, which is on the shoe rack, is four years tomorrow.
In the sentence above, the information of the location of my gold shoe is not important to the sentence. I have only one gold shoe; hence, the extra information does not help to identify it.
The University of Ghana, which is the premier university, is in Accra, Ghana.
Obviously, we do not need the extra information about the University of Ghana to understand the rest of the sentence. The non-restrictive clause could be taken out and you would still understand the sentence.
Let us switch that and which among the sentences we have used so far and see if their meanings will change with each swap.
We start with the sentences that used that. Let us see what happens if we use which instead.
Leigh’s phone (noun) that has the gold band (restrictive clause) was bought at a thrift shop.
Leigh’s phone (noun), which has the gold band (restrictive clause), was bought at a thrift shop.
Changing that to which in the second sentence gives off the implication that Leigh only has one phone.
A cat that adores its owner is usually agreeable to strangers.
A cat, which adores its owner, is usually agreeable to strangers.
The meaning of the sentence has changed to express the notion that all cats are usually agreeable to strangers. Since all cats are not agreeable to strangers, the sentence is false.
We are on to the sentences that used which; this time, we see how their meanings change when we use that.
My gold shoe, which is on the shoe rack, is four years tomorrow.
My gold shoe that is on the shoe rack is four years tomorrow.
In the second sentence, the implication is that I have multiple gold shoes, hence making it prudent to specify the one on the shoe rack.
The University of Ghana, which is the premier university, is in Accra, Ghana.
The University of Ghana that is the premier university is in Accra, Ghana.
Changing which to that in the second sentence implies that there are many universities that are known by the name ‘University of Ghana’, and the sentence specifies that first (premier) one is located in Accra.
