Relative clauses
Relative clauses modify a noun phrase by adding identifying or supplementary information and must attach unambiguously to their antecedent.
01 · Concept foundation
Understand the terms before applying the rule
Each term below names a different grammatical object. Open examples and compare their function rather than memorising a Vietnamese translation alone.
relative clause/ˈrelətɪv klɔːz/
mệnh đề quan hệA clause modifying a noun phrase and usually introduced by who, which, that, whose, where or when.
the station that recorded the peak
trạm đã ghi nhận giá trị đỉnh
defining relative clause/dɪˈfaɪnɪŋ ˈrelətɪv klɔːz/
mệnh đề quan hệ xác địnhA relative clause necessary to identify the intended referent and written without surrounding commas.
The stations that failed were replaced.
Những trạm bị hỏng đã được thay.
non-defining relative clause/nɒn dɪˈfaɪnɪŋ ˈrelətɪv klɔːz/
mệnh đề quan hệ không xác địnhA supplementary relative clause separated by commas and not required to identify the referent.
Station A, which failed, was replaced.
Trạm A, vốn bị hỏng, đã được thay.
Complete lesson scope
Do not stop at one formula
Defining and non-defining relative clauses
Who, whom, whose, which, that, where and when
Subject/object relatives and pronoun omission
Preposition placement, sentential which and reduced relatives
Decision boundary: Place the relative clause next to its antecedent and use commas only for supplementary information.
02 · Controlling rule
A relative clause attaches a proposition to a noun phrase. Defining clauses restrict reference and normally have no commas; non-defining clauses add supplementary information and are separated by commas or pauses. The relative marker has a grammatical role inside the clause. Subject relatives cannot omit the marker, object relatives sometimes can, and reduced relatives use V-ing for active meaning, V3 for passive meaning or to-infinitive for purpose, order or potential.
head noun + who/whom/whose/which/that/where/when + clause | head + V-ing/V3/to-infinitive reduced relativeRelative clauses: identification, addition and controlled reduction
Attach information to the correct noun, distinguish defining from non-defining clauses, select or omit relative markers accurately, and reduce clauses only when reference remains unambiguous.
A relative clause is part of a noun phrase. Its first job is to identify or describe a noun; punctuation and pronoun choice then signal whether the information is essential, additional, subject-like or object-like.
Defining clauses restrict reference; non-defining clauses add parenthetical information. Subject relatives cannot be omitted, object relatives sometimes can, and that is excluded from non-defining clauses. Reduction to V-ing, V3 or to-infinitive is a meaning-preserving transformation, not a mechanical deletion.
Which noun is the antecedent, and is the clause attached directly to it?
Would removing the clause change which person or thing is meant?
Does the relative marker function as subject, object, possessive, place, time or reason?
Can the clause be reduced without changing voice, time relation or subject control?
1. Defining and non-defining information
Defining clauses identify which member of a class is meant. Non-defining clauses assume the reference is already identifiable and add extra information with commas and spoken pauses.
defining: noun + relative clause · non-defining: identifiable noun, relative clause,Defining clauses do not take separating commas because the information is integrated with the noun's identity.
Non-defining clauses require commas in writing and intonational boundaries in speech.
That can replace who/which in many defining clauses but not in non-defining clauses.
The sensors that recorded continuous data were retained.
The sensors that recorded continuous data were retained.
The defining clause selects a subset of sensors.
The ADCP, which was installed in May, failed during the storm.
The ADCP, which was installed in May, failed during the storm.
The device is already identifiable; the clause adds background information.
Defining relative clause
noun + who/which/that/whose/where/when + clauseRestrict reference and identify which member is meant.
the method that produced the lowest error
the researcher whose paper we cited
- No separating commas; object marker may sometimes be omitted.
Non-defining relative clause
identifiable noun, who/which/whose/where/when + clause,Add parenthetical information without changing identity.
The Thames, which flows through London, ...
- Use commas and do not use that.
Reduced relative
noun + V-ing/V3/to-infinitiveCompress a recoverable active, passive or identifying relation.
students studying abroad
data collected in 2025
the first method to converge
- Check voice and implied subject before reducing.
Comma changes reference
The students who submitted early received feedback.
Only the early-submitting students received it.
The students, who submitted early, received feedback.
All the identifiable students submitted early; the clause is additional.
Punctuation encodes whether the clause restricts the reference set.
Subject versus object relative
The model that failed was replaced.
That is subject of failed and cannot be omitted.
The model (that) we tested was replaced.
That is object of tested and may be omitted in a defining clause.
Look for an explicit subject after the relative marker before deciding omission.
Fronted versus stranded preposition
the method on which the analysis relies
Formal written style.
the method the analysis relies on
Neutral and common in speech.
Choose by register; both can be grammatical when attachment is clear.
IELTS Speaking
- Prefer
- Use short who/which clauses to identify people, places and experiences, with natural pauses for non-defining information.
- Avoid
- Whom-heavy or deeply nested structures that interrupt fluency.
- Why
- Relative clauses should extend an answer without overloading working memory.
IELTS Writing Task 2
- Prefer
- Use defining relatives for categories and non-defining relatives for concise background.
- Avoid
- Comma errors that unintentionally claim every member has the same property.
- Why
- Reference precision affects argument accuracy.
Scientific definitions and reporting
- Prefer
- Attach the clause next to its antecedent and reduce only when voice and subject remain recoverable.
- Avoid
- Remote attachment that lets the clause modify the wrong noun.
- Why
- A misplaced relative can change the scientific claim.
Do not translate markers mechanically: reconstruct the relation and viewpoint
Relative marking
Relative pronouns/adverbs and punctuation explicitly mark grammatical role and clause type.
Vietnamese often places a descriptive clause after the noun without an overt equivalent of who/which/that.
When translating into English, identify the missing grammatical role before selecting or omitting a relative marker.
Defining distinction
Comma and intonation can change whether the clause restricts identity.
The same distinction may rely more on discourse context and pause, with less obligatory punctuation signalling.
Do not copy Vietnamese punctuation mechanically; decide whether the English clause is essential or parenthetical.
Relative marker by function
That is restricted to defining clauses; whom is mainly formal/objective.
| Function | Marker | Omission? |
|---|---|---|
| person subject | who/that | no |
| person object | whom/who/that | yes in defining |
| thing subject/object | which/that | object only |
| possession | whose | no |
| place/time | where/when | rewrite possible |
Relative clause reduction map
Reduction is optional and must preserve the intended relation.
| Full clause | Reduced form | Meaning |
|---|---|---|
| people who live nearby | people living nearby | active |
| data that were collected | data collected | passive |
| the first method that achieved convergence | the first method to achieve convergence | order/identification |
✕ The ADCP, that was installed in May, failed.
✓ The ADCP, which was installed in May, failed.
That is not used in non-defining relative clauses.
✕ The sensor recorded the peak was replaced.
✓ The sensor that recorded the peak was replaced.
A subject relative marker cannot be omitted.
✕ The researcher who she designed the survey presented the results.
✓ The researcher who designed the survey presented the results.
Who already functions as subject of designed.
✕ Students, who need support, should contact the tutor.
✓ Students who need support should contact the tutor.
Without context showing all students need support, the clause should define the relevant subgroup.
Choose by relation, viewpoint and discourse role
1. Which sentence means only some sensors were retained?
2. Which relative marker can be omitted?
3. Which reduced form is passive?
4. Which form is appropriate in a non-defining clause?
Create one IELTS Speaking answer using a non-defining relative naturally, one scientific definition using a defining relative, and one reduced relative in a methods sentence. Explain the antecedent and grammatical function in each.
The clause is attached directly to the intended noun.
Comma choice matches defining or non-defining meaning.
The relative marker's subject/object role is correct.
Any reduction preserves voice and reference.
03 · Worked examples
Observe form, function and meaning together
The stations that recorded complete series were retained.
Các trạm ghi được chuỗi đầy đủ đã được giữ lại.
The estuary, which receives strong tidal forcing, requires a finer grid.
Cửa sông, nơi chịu tác động thủy triều mạnh, cần lưới mịn hơn.
The researcher whose dataset was used is listed as a co-author.
Nhà nghiên cứu có bộ dữ liệu được sử dụng được liệt kê là đồng tác giả.
The method on which the analysis relies is documented in Appendix A.
Phương pháp mà phân tích dựa vào được trình bày trong Phụ lục A.
Sensors installed below mean sea level require corrosion protection.
Các cảm biến được lắp dưới mực nước biển trung bình cần được bảo vệ chống ăn mòn.
The first team to complete the survey will validate the metadata.
Nhóm đầu tiên hoàn thành khảo sát sẽ kiểm định siêu dữ liệu.
04 · High-risk contrast
Explain why one form fails, not only which answer is correct
The sensor which it failed was replaced.
The sensor which failed was replaced.
Which already functions as the subject of failed. Adding it creates a second subject. If the relative marker is an object, a separate subject is required and the marker may sometimes be omitted.
05 · Mastery check
Apply the rule before marking the lesson complete
Which sentence is grammatically acceptable in the target system?
Which description best defines “relative clause”?
Which example is one of the verified target patterns in this lesson?
Which structural formula belongs to this lesson?
Complete all four checks, then submit a sentence for target-form feedback.
06 · IELTS Academic
Transfer grammar into a real communicative task
In Speaking, relative clauses let you add identifying or descriptive detail without starting a new sentence; non-defining clauses require a natural pause. In Writing, use them for definitions, classifications and compact evidence, but avoid excessively nested noun phrases. Correct comma use can change meaning, so punctuation is part of grammar rather than decoration.
Distinguish defining and non-defining meaning through reference, commas and intonation.
Use who, whom, whose, which, that, where and when according to grammatical role.
Omit an object relative marker only where grammar and register permit it.
Convert a full relative clause into an accurate V-ing, V3 or to-infinitive reduction.