English Relative Clauses: Who, Which & That Explained | 英文關係子句完整指南

Most Taiwan English learners handle simple sentences with no trouble. The real gap shows up when sentences need to flow together — when one idea must describe or define another. That is the job of relative clauses, and writers who use them well sound dramatically more precise and natural. This guide covers everything: the five key pronouns, the comma rules, the THAT vs. WHICH decision, and the errors Taiwan learners make most often.

What Is an English Relative Clause? [關係子句的基本概念]

A relative clause is a group of words that follows a noun and adds information about it. It always starts with a relative pronoun and functions like an adjective inside a larger sentence.

Here is the difference in practice:

• Without a relative clause: The teacher is helpful.
• With a relative clause: The teacher who helped me pass TOEIC is now retired.

The relative clause “who helped me pass TOEIC” tells you exactly which teacher. Without it, the sentence is vague. With it, you have packed two ideas into one. That efficiency is why relative clauses appear so frequently in academic, business, and everyday English — and why the TOEIC Reading section tests them repeatedly. Every relative clause contains a subject and a verb, but it cannot stand alone as a sentence. It depends on the main clause to give it meaning — in grammar terms, this makes it a dependent clause.

Consistent study habits build the pattern recognition needed to use relative clauses naturally.

The 5 Relative Pronouns You Need [5個必學關係代名詞]

English has five main relative pronouns, each with a specific role. Learning which one to use — and why — removes most confusion instantly.

A quick memory trick for where そして when: if your relative clause modifies a place noun (city, building, country, market), use where. If it modifies a time noun (day, year, moment, season), use when. Neither of these takes a comma in restrictive use — but both can appear in non-restrictive clauses when you are adding bonus information about an already-identified place or time.

Grammar reference books remain one of the fastest ways to build confidence with structures like relative clauses.

Restrictive vs. Non-Restrictive Clauses [限定與非限定關係子句]

This is the distinction that trips up even advanced learners, and it comes down to one question: Is this information essential to the meaning, or just extra detail?

Restrictive clauses (defining clauses) narrow down which specific noun you mean. Remove them and the sentence becomes too vague or loses its point entirely. No commas:

The HSR train that stops at Taoyuan is faster than the local service.

Take out “that stops at Taoyuan” and you no longer know which train. The clause is essential — no commas, because it is defining which subject you mean.

Non-restrictive clauses (non-defining clauses) add bonus information about a noun you have already fully identified. Remove them and the sentence still makes complete sense. Commas required:

The HSR, which opened in 2007, is now used by over 60 million passengers per year. (Wikipedia)

Take out “which opened in 2007” — the sentence works perfectly without it. Bonus detail means commas on both sides of the clause. The comma rule is not optional in formal English. On TOEIC Part 5, this comma distinction often separates the correct answer from the wrong one.

Recognizing restrictive and non-restrictive clauses is a key skill for TOEIC Part 5 and academic writing.

THAT vs. WHICH: The One Decision That Fixes Most Errors [THAT 和 WHICH 怎麼選？]

使用 which where that belongs — or vice versa — accounts for a large portion of relative clause mistakes. The rule is simple once you have the restrictive/non-restrictive framework:

• THAT → restrictive clauses only (no comma before it)
• WHICH → non-restrictive clauses only (always preceded by a comma)

If you can place a comma before your relative clause without changing the meaning, use which. If you cannot, use that.

❌ Wrong: The report, that I submitted yesterday, has an error.
✓ Correct: The report, which I submitted yesterday, has an error.

❌ Wrong: The exam which is on Friday requires advance registration.
✓ Correct: The exam that is on Friday requires advance registration.

Informal American English often blurs this rule — you will see that in both positions in social media and casual speech. In formal writing — business emails, TOEIC tasks, academic essays — the distinction signals grammatical precision. It is one of the clearest markers between intermediate and advanced English production.

Choosing between THAT and WHICH is one of the highest-impact grammar decisions in formal English writing.

WHO vs. WHOM — When Does It Matter? [WHO 還是 WHOM 哪個對？]

Formal English requires whom when the person in the relative clause is the object of a verb or preposition — they receive an action rather than perform one. In everyday conversation, who has largely taken over both roles. But whom still appears in business correspondence, legal documents, and TOEIC answer choices.

The fastest test: substitute he/she または him/her inside the clause. If him fits, use whom.

The client __ I spoke with was satisfied.
→ "I spoke with him" → use whom
✓ The client whom I spoke with was satisfied.

The colleague __ sent the file made a mistake.
→ "彼 sent the file" → use who
✓ The colleague who sent the file made a mistake.

If formal whom feels unnatural in context, restructuring without any relative pronoun is also correct and sounds clean: “The client I spoke with” drops the pronoun entirely, which works in both formal and informal writing.

Relative pronouns appear in every professional English context — mastering them pays off in TOEIC and workplace writing.

5 Mistakes Taiwan Learners Make Most Often [台灣學生最常犯的5個錯誤]

After years of teaching English in Taiwan, these five errors come up most consistently in student writing and speaking — and all of them are straightforward to fix once you know the rule behind them.

1. The double subject
Chinese relative clause structure sometimes leads learners to repeat the subject inside the English clause:

❌ The woman who she lives next door is a nurse.
✓ The woman who lives next door is a nurse.
The relative pronoun replaces the subject. One subject only.

2. Using WHICH for people
Which is never used for people in standard English:

❌ The student which passed the exam is from Tainan.
✓ The student who passed the exam is from Tainan.

3. Missing commas in non-restrictive clauses
If you are adding extra information, commas change the grammatical meaning — they are not decorative:

❌ My boss who grew up in Kaohsiung retired last year. (implies you have multiple bosses)
✓ My boss, who grew up in Kaohsiung, retired last year.

4. Using THAT after a comma
That is never correct in non-restrictive clauses. After a comma, always use which:

❌ Taiwan, that is an island in East Asia, has 23 million residents.
✓ Taiwan, which is an island in East Asia, has 23 million residents.

5. Forgetting WHERE and WHEN
使用 which where where または when should go is a common substitution:

❌ The city which I was born is Hsinchu.
✓ The city where I was born is Hsinchu.

Identifying and correcting errors is how grammar knowledge becomes grammar instinct.

Relative Clauses in TOEIC and Business Writing [TOEIC 與職場英文中的應用]

The TOEIC Reading section tests relative pronouns directly in Part 5 (incomplete sentences) and Part 6 (text completion). The correct answer often depends on whether the clause is restrictive or non-restrictive — which is why understanding the comma rule is actually a test-taking skill, not just grammar theory. Knowing the rule eliminates two or three wrong choices instantly and shaves seconds off your decision time.

In business writing, relative clauses let you deliver dense information efficiently. Instead of writing two separate sentences, you pack context and fact into one:

We have partnered with a consulting firm that specializes in Taiwan market entry to support the expansion.

This reads cleaner than two sentences and signals more confident writing. The British Council’s grammar reference notes that relative clauses appear in roughly one out of every six formal written sentences — making them one of the highest-frequency complex structures in professional English. Reading English business articles and news daily is the fastest way to build the pattern recognition that replaces conscious rule-checking.

Classroom grammar practice builds the foundation; real-world reading and writing makes it automatic.

8 Practice Sentences [8句練習：自我測驗]

Choose the correct relative pronoun. Answers follow.

1. The app __ I use for vocabulary practice was developed in Taiwan.
2. My English teacher, __ has lived in Taipei for 15 years, speaks perfect Mandarin.
3. Taipei 101, __ was completed in 2004, is visible from across the city on clear days.
4. The company __ products I buy most often is headquartered in Hsinchu.
5. I still remember the summer __ I first studied abroad.
6. The colleague __ finished the report first received an early weekend.
7. The café __ we had our first meeting closed last month.
8. The contract, __ was signed last Tuesday, takes effect immediately.

Answers:

1. that (or which) — restrictive clause, thing
2. who — non-restrictive clause, person (commas required)
3. which — non-restrictive clause, thing (commas required)
4. whose — possession
5. when — time noun
6. who — subject position, person
7. where — place noun
8. which — non-restrictive clause, thing (comma before clause)

The most effective way to internalize relative clauses is to notice them in English you already read — news articles, work emails, TOEIC practice passages. Start marking every relative clause you find. Within two weeks, the pronouns and commas will start placing themselves correctly without conscious effort. For more grammar that directly improves your TOEIC score, read our guide on Gerunds vs. Infinitives: 7 Rules Taiwan Learners Must Know. And if you want to build the connected, fluent sentences that high-scoring writers produce, our 50 Most Common Phrasal Verbs Guide is the natural next step.

Sources [參考資料]

1. British Council — Relative Clauses Grammar Reference — Comprehensive guide to defining and non-defining relative clauses with practice exercises.
2. BBC Learning English — Relative Clauses Unit — Intermediate-level breakdown with listening and grammar practice.
3. Wikipedia — Taiwan High Speed Rail — Factual source for the Taiwan HSR example used in the article.
4. ETS — About TOEIC — Official reference for TOEIC test format and Reading section structure.