Relative Clauses: Defining vs Non-Defining (Complete Guide + Examples)

One misplaced comma can completely flip the meaning of a sentence. Learners from Bangkok, Seoul, Ho Chi Minh City, Tokyo, Singapore, and Mexico City all face the same dilemma — do I put a comma before "who" or not? The short answer is: it depends on whether your relative clause is defining or non-defining.

Relative clauses in English come in two types. Defining clauses identify which person or thing you mean — no commas, essential to the sentence. Non-defining clauses add bonus information about a noun that is already identified — commas always required, never essential. Master this single distinction and you gain precise control over a grammar structure that appears in TOEIC Part 5, IELTS Writing Task 2, university essays, and professional emails every day.

This guide covers everything: the rules, real examples, how to choose the right relative pronoun, the most common errors learners worldwide make, exam strategies, and practice exercises. By the end, you will know exactly which type to use — and why the comma makes all the difference.

What Is a Relative Clause in English? (Quick Reference)

Direct Answer: A relative clause is a dependent clause that begins with a relative pronoun — who, which, that, whose, where, when, or why — and gives more information about a noun in the main sentence. There are two types: defining (no commas, essential to meaning) and non-defining (always with commas, adds extra information).

The noun that the relative clause modifies is called the antecedent.

Example: The teacher who inspired me changed my life. → "The teacher" = antecedent | "who inspired me" = relative clause

Think of relative clauses as attached information tags. They answer reader questions like Which one? What kind? Tell me more. They appear in academic writing, business communication, spoken conversation, and exam papers at every level. For a complete picture of how relative clauses sit within the broader grammar system, the Complete English Grammar Guide at EnglishPractices.com maps every core structure by level and function — a strong starting point for any learner building grammar confidence from the ground up.

Defining Relative Clauses: The Clause You Cannot Remove

The Core Rule

A defining relative clause — also called a restrictive clause — tells the reader which specific person, place, or thing you mean. It is essential to the identity of the noun. Remove it, and the sentence becomes too vague, too broad, or completely unclear.

No commas are used in defining relative clauses. Ever.

If removing the clause makes the sentence unclear or too broad → it is defining. Use no commas.

Examples That Show Why This Matters

In every case, the relative clause does essential identification work. Without it, the sentence loses its core meaning.

Which Relative Pronouns Can You Use in Defining Clauses?

• Who / that → for people: The engineer who designed this bridge studied in the UK.

• Which / that → for things or animals: The software that we use is being updated.

• Whose → for possession (people, things, or animals): The writer whose book I read has a new series out.

• Where → for places: This is the café where I first tried Vietnamese coffee.

• When → for times: There are mornings when nothing seems to work.

• Why → for reasons: That is the exact reason why she left the company.

Exam strategy: In defining relative clauses, "that" can replace "who" or "which" in informal and semi-formal English. However, for IELTS Writing Tasks and formal academic or business English, "who" and "which" are the preferred forms.

Non-Defining Relative Clauses: The Clause You Could Drop

The Core Rule

A non-defining relative clause — also called a non-restrictive clause — adds extra information about a noun that the reader can already identify clearly. This information enriches the sentence, but it is not essential to identify the noun. Because it functions as a parenthetical aside, commas are always required, both before and after the clause when it appears in the middle of a sentence.

If removing the clause leaves the sentence perfectly clear and the noun fully identified → it is non-defining. Always use commas.

Examples That Make the Rule Concrete

The clause adds colour, depth, and context. But the noun is already fully identified without it.

The "That" Rule — Non-Negotiable

This is one of the most frequently tested points in TOEIC Part 5 and IELTS grammar tasks:

You cannot use "that" in a non-defining relative clause. No exceptions.

❌ My manager, that started last month, has already restructured the team. ✅ My manager, who started last month, has already restructured the team.

❌ The Great Wall, that stretches over 21,000 kilometres, is a UNESCO World Heritage Site. ✅ The Great Wall, which stretches over 21,000 kilometres, is a UNESCO World Heritage Site.

Defining vs Non-Defining: The Side-by-Side You Need

Head-to-head with the same noun — watch how the comma shifts meaning:

• Defining: Teachers who give regular feedback produce stronger students. → We mean a specific group: teachers who give feedback. Teachers who do not give feedback are excluded from this claim.

• Non-Defining: Ms. Park, who gives regular feedback, is one of the best teachers in the programme. → Ms. Park is already identified by name. The clause adds a bonus fact about her.

The comma is not decorative. It signals to your reader whether the clause is essential or supplementary — and getting this wrong in academic writing or professional communication creates unintended meaning.

Relative Pronouns: The Full Reference You Will Actually Use

Choosing the correct relative pronoun is the second major challenge after understanding the comma rule.

On "whom": In spoken English, many native speakers use "who" in place of "whom" — and that is widely accepted in informal contexts. In formal written English, however, "whom" is the correct object form, particularly after prepositions: to whom, by whom, for whom. For IELTS Academic Writing, using "whom" correctly signals grammatical sophistication.

Understanding how relative pronouns interact with prepositions is part of a larger skill set. The guide to Prepositions of Time, Place, and Movement at EnglishPractices.com explains how prepositions work alongside pronouns and clause structures in formal English sentences.

Five Mistakes That Almost Every English Learner Makes

Learners from different language backgrounds make predictable, consistent errors with relative clauses. Here are the five most common — and how to fix each one.

1. Adding a Subject Pronoun After the Relative Pronoun (Resumptive Error)

❌ The scientist who he discovered the vaccine won a global award. ✅ The scientist who discovered the vaccine won a global award.

This error is common among learners from Thailand, Vietnam, Korea, and Arabic-speaking countries, whose native languages structurally require a pronoun to remain in the clause. In English, the relative pronoun already replaces the subject — no double subject.

2. Using "That" in a Non-Defining Clause

❌ Seoul, that is the capital of South Korea, hosted a major international summit. ✅ Seoul, which is the capital of South Korea, hosted a major international summit.

3. Forgetting the Second Comma

❌ My colleague, who joined last month works extremely hard. ✅ My colleague, who joined last month, works extremely hard.

Non-defining clauses embedded mid-sentence need a comma before and after. Missing the closing comma is among the most common punctuation errors in student writing globally.

4. Using "Which" for People

❌ The manager which interviewed me was very professional. ✅ The manager who interviewed me was very professional.

"Which" is for things and animals. "Who" is for people. This rule has no exceptions in standard English.

5. Dropping the Relative Pronoun When It Is the Subject

You can omit a relative pronoun in a defining clause — but only when it is in the object position (i.e., the clause already has its own stated subject).

✅ The book (that) I read was excellent. — "that/which" is the object; omission is grammatically fine. ❌ The man called me yesterday is my former professor. — "who" is the subject; you cannot drop it.

How Relative Clauses Appear in TOEIC and IELTS

TOEIC Part 5 — Incomplete Sentences

TOEIC Part 5 tests relative pronoun selection directly. You will see questions in this format:

The employee _____ receives the highest performance rating will be promoted to team leader. (A) who (B) which (C) whose (D) whom

Answer: (A) who — "employee" is a person; the pronoun is the subject of the clause.

TOEIC tests your ability to: select the correct relative pronoun, distinguish defining from non-defining structures, and identify when "that" is or is not appropriate.

IELTS Writing Task 2 — Grammatical Range and Accuracy

The IELTS official band descriptors, published by the British Council and IDP, state that a Band 7 candidate "uses a variety of complex structures with some flexibility and accuracy." Non-defining relative clauses are among the most effective ways to demonstrate that complexity without overcomplicating your argument.

Example of a Band 7 sentence: "Governments, which are responsible for public health infrastructure, should allocate greater funding to preventive care programmes."

Relative clauses also combine powerfully with other complex structures. The lesson on Conditionals in English and the guide to Reported Speech Rules at EnglishPractices.com pair naturally with relative clauses to give your IELTS Writing the structural variety that separates Band 6 from Band 7 and above.

Advanced Use: Relative Clauses Combined With Passive Voice

In academic writing, business reports, and IELTS Task 1 responses, relative clauses frequently combine with the passive voice to produce authoritative, precise sentences.

The proposal that was submitted by the research team has been approved by the board. The regulations, which were introduced across the EU in 2023, significantly changed corporate reporting requirements.

Both structures appear in formal written English as standard practice. The Passive Voice Guide at EnglishPractices.com shows exactly how to build passive structures across tenses — a strong companion skill for anyone working on academic or professional writing.

Relative Clauses and Tense Combinations: A Quick Framework

Relative clauses work across all tenses and frequently combine with other grammar structures to produce layered, nuanced sentences. Two combinations worth knowing:

1. Relative clause + present perfect: The scientist who has discovered a treatment for the condition is based in South Korea. The present perfect inside the relative clause signals recent relevance. The Present Perfect Tense guide at EnglishPractices.com explains exactly when and how to make this combination work.

2. Relative clause + modal verbs: Students who can speak English fluently will have a measurable advantage in global job markets. Modal verbs inside relative clauses express ability, possibility, or obligation about the antecedent. The Modal Verbs Guide at EnglishPractices.com covers "can," "could," "should," "must," and more with real-world context.

2026 Insight: Why Relative Clauses Are More Valuable Than Ever

In 2026, with AI writing assistants embedded in many learners' daily workflows, a clear pattern has emerged in both academic and professional writing: AI-generated text tends to default to simple, declarative sentence structures. Non-defining relative clauses — the very structure that gives experienced writers nuance and texture — are consistently underused in AI-assisted drafts.

This means learners who can deploy defining and non-defining relative clauses accurately and naturally stand out immediately. To IELTS examiners, to university professors reviewing applications, and to hiring managers reading cover letters, correct relative clause usage is a visible marker of genuine grammatical command — not a performance generated by a tool. Knowing and applying these rules gives you a strategic edge that AI assistance alone cannot replicate.

Quick-Reference Practice Exercises

Identify each clause as defining (D) or non-defining (ND), and add commas where necessary:

1. The city where the summit will be held is Geneva.

2. Nelson Mandela who became South Africa's first democratically elected president served from 1994 to 1999.

3. The laptop that I use for work runs the latest design software.

4. My grandmother who turned 85 last month still walks to the market every morning.

5. The report which the team submitted on Friday has been approved.

Answers:

1. D — no commas. "Where the summit will be held" identifies which city.

2. ND — commas required: Nelson Mandela, who became South Africa's first democratically elected president, served from 1994 to 1999.

3. D — no commas. "That I use for work" identifies which laptop.

4. ND — commas required: My grandmother, who turned 85 last month, still walks to the market every morning.

5. Depends on context. If "the report" is already known to both parties → ND with commas. If several reports exist and this clause identifies the right one → D without commas.

The EnglishPractices.com Approach to Grammar Mastery

EnglishPractices.com has spent over 19 years helping learners across Vietnam, Thailand, South Korea, Japan, Singapore, Mexico, the UK, and beyond move from grammar confusion to genuine fluency. Grammar points like relative clauses sit inside a larger, interconnected framework — and the approach at EP has always been to teach them in context, not in isolation.

The philosophy at EnglishPractices.com is straightforward: clarity first, rules second, and always with examples that reflect real communication — not just exam scenarios. The Complete English Grammar Guide connects every structure — relative clauses, conditionals, passive voice, tense sequencing, articles — into a single coherent learning map. Students from a TOEIC prep class in Seoul and a university student in Ho Chi Minh City are working through the same structured path, supported by 19+ years of teaching methodology refined across global classrooms.

This is the difference between collecting rules and actually building the grammar instinct that makes English feel natural.

Conclusion: Two Rules. Unlimited Sentences.

Relative clauses are not complicated once you see the logic:

1. Defining clause — the clause identifies the noun → no commas, can use "that"

2. Non-defining clause — the clause adds extra info about an already-identified noun → always use commas, never use "that"

Apply these two rules consistently and you will write with the kind of precision that earns marks in IELTS, clears TOEIC Part 5 grammar questions, and impresses readers in any professional or academic context worldwide. Grammar is not about memorising endless lists — it is about understanding the underlying logic so the right choice becomes automatic.

Your next step: Explore the full grammar library at EnglishPractices.com — from Modal Verbs to Present Perfect Tense — and build real grammar confidence, one structure at a time. Have a specific exam target or level in mind? Reach out to the team at EnglishPractices.com and get guidance built around where you are and where you want to go.