English Relative Clauses: A Complete Guide to Using Who, Which, That, and Whom

A relative clause is a subordinate clause that describes a noun and begins with a relative pronoun such as who, which, that, whom, or whose. It lets you combine two sentences into one smooth, specific sentence — for example, "The student who sits in front scored highest." This guide explains each relative pronoun, the difference between defining and non-defining clauses, when to use that versus which, and the mistakes ESL writers make most often.

What Is a Relative Clause?

A relative clause works like an adjective: it gives the reader more information about a noun (the "antecedent"). Without relative clauses, you would have to write many short, separate sentences, which makes your writing feel choppy and repetitive.

Compare these two versions:

The relative clause "who works at the hospital and helped me yesterday" tells us which woman, and it does so without starting a brand-new sentence. Native speakers use relative clauses constantly in both speech and writing, which is exactly why mastering them makes your English sound natural and fluent.

Every relative clause has two essential parts: a relative pronoun (who, which, that, whom, whose, where, when, why) and a subject-verb relationship of its own. "Who works at the hospital" has the subject "who" and the verb "works." If you remove the relative pronoun, the clause loses its connection to the noun it describes, so the pronoun is never optional in standard writing.

Relative clauses appear after the noun they modify. They can describe people, things, places, times, or reasons, and the pronoun you choose depends on what the noun is and what kind of information you want to add.

Relative Pronouns at a Glance

Here is the quick-reference map most ESL writers need. Keep it open while you write, and the rest of this guide will explain each row in detail.

Who and Whom

Who is the subject of the relative clause (it does the action). Whom is the object (it receives the action). In everyday writing, many native speakers replace "whom" with "who" in spoken English, but in formal writing you should keep the distinction.

A simple test: if you could replace the pronoun with he/she, use who; if you could replace it with him/her, use whom. "You praised him" → "whom." This small habit removes one of the most visible errors in ESL academic writing.

Which

Use which for things and animals. In American English, which is strongly preferred in non-defining clauses (the ones with commas). In British English, which also appears in defining clauses more freely. Either way, which never refers to people.

That

Use that for both people and things, but only in defining clauses (no commas). "The chair that broke was old." You cannot use that in a non-defining clause — "My dog, that is brown, is friendly" is incorrect; it must be "My dog, which is brown, is friendly."

Whose

Whose shows possession and works for people and things, unlike in some languages where a different word is used for objects. "The company whose product failed recovered quickly." Do not confuse it with "who's" (who is) — that is a different word entirely.

Where, When, Why

These relative words replace "preposition + which" in a cleaner way. "The office in which I work" becomes "The office where I work." "The day on which we met" becomes "The day when we met." "The cause for which he quit" becomes "The reason why he quit." Using the shorter form is almost always better for readability.

Defining vs Non-Defining Relative Clauses

This is the single most important distinction, and the one that changes meaning. A defining (restrictive) clause identifies which noun you mean. Without it, the sentence is unclear. A non-defining (non-restrictive) clause simply adds extra information; the sentence still makes sense without it.

The punctuation rule is strict: non-defining clauses always take commas, and you must use which (not that) for things. Defining clauses take no commas and can use that or which. Getting this wrong can actually change your meaning, which is why exams and editors watch it closely.

When to Use "That" vs "Which"

Many ESL writers freeze at this choice. The good news is that the rule is simpler than it looks once you separate the two clause types.

In defining clauses: both that and which are grammatically correct for things in most varieties of English. American English slightly prefers that ("The report that you filed"), while British English accepts both. For people in a defining clause, use who or that ("The student that/who won").

In non-defining clauses: you must use which for things, never that. "Our office, which is in Shanghai, is busy" is correct; "Our office, that is in Shanghai, is busy" is wrong.

If you want one safe default: use that for defining clauses about things, and which (with commas) for non-defining clauses. This matches standard American academic style and avoids the most common errors.

Common Relative Clause Mistakes ESL Writers Make

After reviewing thousands of ESL texts, we see the same relative-clause errors repeat. Fix these and your writing improves immediately.

1. Dropping the relative pronoun incorrectly

In some languages you can omit the relative word; in English you usually cannot in formal writing. "The book I read it was good" is wrong — the "it" is a duplicate subject. Write "The book that I read was good" (or "The book I read was good" in informal contexts, but keep the pronoun in academic work).

2. Using "which" for people

"The girl which won" is incorrect. People take who/that; things take which/that. This is one of the fastest ways to spot non-native writing.

3. Using "that" in a non-defining clause

As noted above, "My car, that is red, is fast" must be "My car, which is red, is fast." Pair that with no commas only.

4. Adding a second subject by mistake

After the relative pronoun, do not repeat the noun as the subject. Wrong: "The manager who he hired me." Right: "The manager who hired me." The pronoun "who" already stands in for "the manager."

5. Wrong comma placement

Putting commas around a defining clause breaks the meaning. "Students, who study daily, improve" wrongly implies all students study daily and you are just adding detail. Without commas — "Students who study daily improve" — you correctly mean only the daily studiers improve.

6. Preposition at the wrong end (or missing it)

In formal English you may write "The person with whom I work," but in normal writing "The person who I work with" is acceptable. What is wrong is dropping the preposition entirely: "The person who I work" leaves the reader hanging. Keep the preposition, wherever you place it.

These patterns connect directly to broader grammar habits. If relative clauses feel shaky, review our guide on common English grammar mistakes and how to fix them, and our 10 most common grammar mistakes ESL learners make for the full picture.

Relative Clauses and Sentence Flow

Relative clauses are not just grammar exercises — they are a core tool for sentence variety. A paragraph made only of short sentences ("I went to the store. The store was busy. I left.") reads like a list. Joining ideas with relative clauses creates rhythm:

But balance matters. Stacking too many relative clauses in one sentence creates a "noun-clause jungle" that hurts readability. A good rule: use at most one relative clause per sentence in most contexts, and break longer thoughts into two sentences when the clause gets complicated. For deeper work on mixing sentence types, see our article on how to improve English sentence structure.

Relative clauses also pair well with the verb tenses you choose. A clause in the past ("the email that arrived yesterday") versus the present ("the email that arrives daily") changes the time frame of your whole sentence, so keep tenses consistent across the main and relative clauses. Our complete guide to English verb tenses explains how to keep them aligned.

Practice With Free Tools

The fastest way to lock in relative clauses is to write your own sentences and get instant feedback. ClearWrite Tools gives you three free helpers:

• Grammar Checker — paste a paragraph and spot missing relative pronouns, wrong who/which choices, and comma errors in seconds.
• Readability Checker — if a relative clause makes a sentence too long, the tool flags it so you can split it for clarity.
• Word Counter — track length as you practice combining sentences, and aim for natural, varied paragraphs.

Make it a daily habit: take three short sentences, merge them with a relative clause, then run them through the grammar checker. Within a week the pattern becomes automatic.

Relative clauses are a small grammar point with an outsized impact on how fluent your writing sounds. Learn the pronouns, respect the comma rule between defining and non-defining clauses, and practice merging sentences daily. Pair that with the free tools above, and your English will read like it was written by a confident, native-level writer.