Why Do We Sometimes Use a Preposition Before “Which”?

Relative Pronouns

“Which,” “who,” and “that” are common relative pronouns. They introduce relative clauses that add information to something that comes before them in a sentence. Look at the following sentence:

We need to study hard for TOEFL which is one of the most famous standardized English tests.

This sentence can be broken down into several parts:

• Subject: We
• Verb: need to study
• Adverbial phrase: hard for TOEFL
• Relative clause: which is one of the most famous standardized English tests

In this sentence, the relative clause comes after the noun “TOEFL.” This means that the relative clause adds information to that noun. The clause tells us that TOEFL is a famous standardized English test.

Structure of Relative Clauses

A relative clause is a type of dependent clause. This means that it has its own subject-verb combination, but it can’t stand alone as a complete sentence. It must be attached to an independent clause.

Let’s look at a sentence to check these rules.

I met a student who studies biology.

• Subject: I
• Verb: met
• Object: a student
• Relative clause: who studies biology

If we break down the relative clause, we get the following:

• Subject: who (meaning “a student”)
• Verb: studies
• Object: biology

In this sentence, the relative clause “who studies biology” has its own subject-verb combination. The subject is “who,” and the verb is “studies.” This shows that it’s a clause, not a phrase. However, it’s not an independent clause because it can’t stand alone as a complete sentence. It depends on the independent clause “I met a student.” Since the relative clause is attached to that independent clause, the whole sentence is complete and grammatical.]

Here’s another example.

I bought a laptop which runs very fast.

• Subject: I
• Verb: bought
• Object: a laptop
• Relative clause: which runs very fast

If we break down the relative clause, we get the following:

• Subject: which, meaning “a laptop”
• Verb: runs
• Adverbial phrase: very fast

In this sentence, “which runs very fast” gives extra information about “a laptop.” Inside this relative clause, “which” points back to “a laptop,” and “runs” is the verb.

This clause can’t be used as a sentence by itself. If we only write “which runs very fast,” the reader won’t know what “which” refers to. That is why the clause needs the independent clause “I bought a laptop.” Together, they form a complete sentence.

When a Preposition Is Used Before “Which”

There are three common relative pronouns:

• Who
• Which
• That

Among them, only “which” is sometimes used with a preposition before it. This may look confusing at first, but the rule is simple. Take a look at the following two sentences:

This is the room in which the meeting took place.

This is the room which the students cleaned.

Why does the first sentence need “in” before “which,” while the second sentence doesn’t?

As we saw earlier, a sentence with a relative clause can be divided into an independent clause and a relative clause. If we separate the first sentence into two ideas, we get the following:

• First clause: This is the room.
• Second clause: The meeting took place in the room.

In the second clause, the phrase “in the room” is necessary because the meeting happened inside the room. When we change “the room” into “which,” the preposition “in” remains. That’s why the sentence becomes “This is the room in which the meeting took place.” Now let’s look at the second sentence.

• First clause: This is the room.
• Second clause: The students cleaned the room.

In this case, “the room” is the object of the verb “cleaned.” We don’t say “cleaned in the room” if we mean that the students cleaned the room itself. Because there is no preposition before “the room” in the original idea, we don’t need a preposition before “which.”

The Key Is the Verb Used in the Relative Clause

Grammatically speaking, whether a relative pronoun needs a preposition before it often depends on the verb used in the relative clause.

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If the main verb in the relative clause is transitive, “which” can work as its object. In that case, “which” is used without a preposition. However, if the main verb in the relative clause is intransitive, “which” can’t work as its direct object. In that case, a preposition is often needed to connect “which” to the verb.

This difference occurs because transitive verbs can take a direct object, while intransitive verbs can’t. For example, we can say “cleaned the room” because “clean” is a transitive verb. However, we can’t say “the meeting took place the room.” We need a preposition: “the meeting took place in the room.” That’s why the relative clause uses “in which.”

This is the issue about which the committee argued.

• Relative clause: about which the committee argued
• Original idea: The committee argued about the issue.

In the original idea, the verb “argued” doesn’t take “the issue” as a direct object in this meaning. We don’t say “The committee argued the issue” when we mean that they discussed it with disagreement. We say “argued about the issue.” Because the preposition “about” is needed in the original idea, it remains before “which” in the relative clause.

Only “Preposition + Which” Can Be Replaced by “Where”

Many learners think that “where” can be used whenever the noun before it refers to a place. However, this is not always true. “Where” can replace “preposition + which” only when the relative clause uses the place as a location.

Compare the following two sentences:

I stayed at the hotel which my friend recommended.

I stayed at the hotel where my friend worked.

At first, both sentences may look similar because both have the place noun “hotel” before the relative clause. However, only the second sentence can use “where.” In the second sentence, “where” means “at which.”

I stayed at the hotel where my friend worked.
= I stayed at the hotel at which my friend worked.

This works because the original idea is “my friend worked at the hotel.” The preposition “at” is necessary, so “at which” can be replaced by “where.”

However, the first sentence is different.

I stayed at the hotel which my friend recommended.

This comes from “my friend recommended the hotel.” Here, “the hotel” is the object of “recommended,” so there is no preposition before it. Since there is no “at which” or “in which,” we can’t use “where.” That is why the following sentence is incorrect:

× I stayed at the hotel where my friend recommended.

The key point is simple: “where” can replace “preposition + which,” such as “in which” or “at which.” It can’t replace “which” when “which” is the object of a verb.