First, let’s identify exactly what a relative pronoun is. In English, there are 3 main relative pronouns common in speech and writing: who, which, and that.
That is by far the most commonly used relative pronoun in speech as well as writing. However, that is also one of the most misused and overused words in writing. That is considered more colloquial or informal than which, and when we use a word in conversation, we automatically insert the more commonly used word in writing. Which is a more formal relative pronoun often used in academic writing, and less often used in day-to-day speech.
When to use each is influenced by a number of factors, but in general, that is used with restrictive relative clauses, while which is used with both restrictive and non-restrictive clauses. Who is a personal pronoun used when referring to a person. Inserting that when who should be used when referring to a person is one of the most common errors related to relative pronoun use, though many writers and even instructors believe it to be more of a judgment call. However, one must know the rule first before breaking it.
It’s usually easy to distinguish when to use who (or whom, whose). In the examples below, the relative pronoun who always refers to a person or people:
The woman who painted the portrait is waiting in the lobby.
The playoffs were great for the fans, whose support never wavered.
The chef, who has a new cookbook, is on the Morning Show tomorrow.
The use of that and which is a bit more complicated, and often simply a matter of preference. That is considered a general use restrictive relative pronoun. The term “restrictive” means that the clause is essential to the meaning of the sentence, as it limits, or restricts, the noun or thing it refers to.
The truck that I want is too expensive.
In the previous sentence, the clause “that I want,” further defines the noun clause, “The truck”; it limits which truck the speaker is referring to.
The boots that are red leather are the only ones I want.
In this example, we wouldn’t know which boots the speaker is referring to without the clause “that are red leather.”
So when considering restrictive relative pronouns, if the clause “restricts” or limits the meaning of preceding noun clause, it is a restrictive relative pronoun.
The previous examples show that used with inanimate nouns, but that can also be used with animate (living/human) nouns, and are considered flexible:
She is the girl that lives down the road.
He is one of the boys that always wants to play soccer at recess.
Review: Both of the previous examples are restrictive clauses, meaning the relative clause (that. . ._) limits the meaning of the preceding noun clause by offering identifying information for “she” and “he.”
A non-restrictive clause is considered “non-essential”; this means that the relative clause offers additional information that the reader doesn’t really need to identify the subject. It’s info that’s nice to have, but not necessary to identify the subject of the noun clause.
Ernest Hemingway, who wrote The Sun Also Rises, is considered a minimalist writer.
He looked into her files, which she never locked.
The airline, which added flights to Omaha, is seeing an increase in passengers.
Each of the noun subjects do not need the additional clause to identify who or what they are. If you lift out the italicized clause, the sentence’s meaning is retained:
Ernest Hemingway is considered a minimalist writer.
He looked into her files.
The airline is seeing an increase in passengers.
In speech, you might notice that the use of “that” as a relative pronoun is by far more common than “which.” However, in written or academic prose, “which” is much more common. So how do you know when to use each?
If removing the words would change the meaning of the sentence, use that.
If removing the words would not change the meaning of the sentence, use which.
Let’s look at the following examples:
Trucks that have hybrid technology are gaining popularity.
If you ask yourself, do all trucks have hybrid technology? the answer would be no. So we need to have that have hybrid technology to know which trucks we’re talking about. In this case, “that” is the correct relative pronoun.
I signed the contract, which was sent certified mail.
The non-restrictive clause, which was sent certified mail, isn’t limiting the noun clause or identifying it in any way. It’s simply offering the reader some extra, interesting information. In this case, “which” is the correct relative pronoun.
If you’re not sure when to use that, which or who, follow these guidelines and you’ll be right most of the time:
Who = (whom, whose) refers to a person or people.
That = Introduces a restrictive clause.
Which = Introduces a non-restrictive clause offering extra information. You can omit the “which” clause with no harm done.