Sentence Structure and Clauses

Lesson 5: Relative Clauses — Defining and Non-Defining

Lesson: 5 of 9 | Level: 🟠 Intermediate — 🟣 Upper-Intermediate

1. Lesson Overview

Relative clauses are subordinate clauses that modify nouns — they identify, define, describe, or add information about a noun or noun phrase in the main clause. They are one of the most frequently used and most important structures in English, appearing constantly in academic, journalistic, and formal writing, and in everyday speech. Getting them right — choosing the correct relative pronoun, punctuating them correctly, and understanding the critical distinction between defining and non-defining relative clauses — is one of the hallmarks of grammatical accuracy at intermediate and advanced level.

This lesson examines both types of relative clause in full — their formation, their relative pronouns, their punctuation, the contexts in which the relative pronoun can be omitted, and the many subtleties of usage that distinguish accurate and sophisticated use from the errors that are most commonly made.

Objectives

By the end of this lesson, you will be able to:

• Distinguish defining and non-defining relative clauses and punctuate them correctly
• Use the correct relative pronoun — who, whom, whose, which, that — in all contexts
• Understand when the relative pronoun can be omitted
• Form and use preposition + relative pronoun constructions in formal writing
• Recognise and correct common errors in relative clause formation and use

2. Core Content

A. What Is a Relative Clause?

A relative clause modifies a noun or a noun phrase in the main clause — adding information about it. The noun or noun phrase that the relative clause modifies is called the antecedent.

The relative clause is introduced by a relative pronoun — who, whom, whose, which, or that — or by a relative adverb — where, when, or why — that connects the relative clause to its antecedent and simultaneously performs a grammatical function within the relative clause itself.

For example:

The scientist who discovered hydrothermal vents received international recognition.

Here, who discovered hydrothermal vents is a relative clause modifying the scientist (antecedent). Who is the relative pronoun — it refers back to the scientist and functions as the subject of discovered within the relative clause.

B. The Two Types of Relative Clause

The most fundamental distinction in the relative clause system is between defining relative clauses (also called restrictive relative clauses) and non-defining relative clauses (also called non-restrictive relative clauses). This distinction is not merely grammatical — it is a difference in meaning and communicative function.

Defining relative clauses

A defining relative clause identifies which person, thing, or group is being referred to — it restricts the meaning of the antecedent to a specific subset. Without the defining clause, the sentence would be incomplete or its meaning would be unclear — the clause is essential to the meaning.

For example:

The species that was discovered in 1977 has never been found anywhere else on Earth.

Without that was discovered in 1977, the sentence would read The species has never been found anywhere else — but which species? The defining clause identifies exactly which species is meant. It is essential to the meaning.

Scientists who study hydrothermal vents often work in conditions of extreme difficulty.

Without who study hydrothermal vents, the sentence would be Scientists often work in conditions of extreme difficulty — a different, much broader claim. The clause is defining — it restricts scientists to a specific group.

Key features of defining relative clauses

• No commas — the relative clause is not separated from its antecedent by commas
• That can be used for both people and things — though who is preferred for people
• The relative pronoun can be omitted when it is the object of the relative clause

Non-defining relative clauses

A non-defining relative clause adds extra information about the antecedent — but this information is not essential to identify which person or thing is meant. The antecedent is already fully identified, and the relative clause simply provides additional, supplementary detail. If the clause were removed, the main meaning of the sentence would remain intact.

For example:

The hydrothermal vent community, which was discovered in 1977, transformed our understanding of life on Earth.

The hydrothermal vent community is already identified — which was discovered in 1977 adds additional information but is not essential to identify which vent community is meant. The sentence would be complete and meaningful without it: The hydrothermal vent community transformed our understanding of life on Earth.

Professor Chen, who has spent her career studying vent ecosystems, received the international science prize.

Professor Chen is already identified by name — who has spent her career studying vent ecosystems adds information but does not define which Professor Chen is meant.

Key features of non-defining relative clauses

• Always enclosed in commas — the clause is separated from its antecedent by commas
• That is never used — who, whom, whose, and which only
• The relative pronoun can never be omitted
• Can refer to an entire clause or proposition — not just a noun

C. Relative Pronouns — Forms and Functions

Who vs. whom

Who is used when the relative pronoun is the subject of the relative clause. Whom is used when the relative pronoun is the object of the relative clause or follows a preposition.

For example:

The scientist who made the discovery (who = subject of made) The scientist whom the committee honoured (whom = object of honoured) The researcher to whom the award was given (whom after preposition to)

In informal speech and writing, who is widely used in both subject and object positions — whom is restricted to formal and academic writing. However, after a preposition, whom is required in all registers in standard British English.

Which vs. that

Which is used in both defining and non-defining relative clauses referring to things. That is used only in defining relative clauses — never in non-defining ones.

For example:

The species that was discovered in 1977 is unique. (defining — that acceptable) The species which was discovered in 1977 is unique. (defining — which also acceptable) The vent community, which was discovered in 1977, is still studied. (non-defining — which only; that not possible) The vent community, that was discovered in 1977, is still studied. ❌ (non-defining — that never used)

In formal and academic British English, which is generally preferred over that in defining relative clauses referring to things — though that is widely used in informal and American English.

Whose

Whose is the possessive relative pronoun — used for both people and things.

For example:

The researcher whose findings transformed the field retired last year. The ecosystem whose diversity continues to astonish scientists was first observed in 1977.

In formal writing, of which can be used instead of whose for things — though whose is now widely accepted for both.

The ecosystem the diversity of which continues to astonish scientists was first observed in 1977. (very formal) The ecosystem whose diversity continues to astonish scientists was first observed in 1977. (preferred)

D. Omitting the Relative Pronoun — Contact Clauses

In defining relative clauses, the relative pronoun can be omitted when it is the object of the relative clause — not when it is the subject. This produces what is sometimes called a contact clause.

When omission is possible — object position

The species that the team discovered was extraordinary. → The species the team discovered was extraordinary. ✅ The methodology (which) scientists have used for decades is being updated. ✅ The researcher (whom) the committee honoured retired last year. ✅

When omission is not possible — subject position

The species that was discovered in 1977 is unique. ❌ → The species was discovered in 1977 is unique. (impossible — that is subject of was discovered) The scientist who made the discovery is still active. ❌ → The scientist made the discovery is still active. (impossible — who is subject of made)

When omission is not possible — non-defining clauses

The relative pronoun can never be omitted in non-defining relative clauses.

The vent community, which was discovered in 1977, is still studied. ✅ The vent community, discovered in 1977, is still studied. ✅ (but this is a reduced relative clause — different structure) The vent community, was discovered in 1977, is still studied. ❌

E. Preposition + Relative Pronoun

In relative clauses where the relative pronoun is the object of a preposition, the preposition can appear in two positions — at the end of the relative clause (informal) or before the relative pronoun (formal).

Informal — preposition at end

The site that the vents were discovered at is now protected. The researcher who the award was given to retired last year.

Formal — preposition before relative pronoun

The site at which the vents were discovered is now protected. The researcher to whom the award was given retired last year.

In formal and academic writing, the preposition + relative pronoun construction is strongly preferred. Note that when the preposition precedes the relative pronoun, that cannot be used — only which or whom.

The site at which the vents were discovered. ✅ The site at that the vents were discovered. ❌

F. Non-Defining Relative Clauses Referring to Whole Clauses

A non-defining relative clause introduced by which can refer not to a single noun but to the entire proposition of the main clause — commenting on the main clause as a whole. This is sometimes called a sentential relative clause.

For example:

The team completed the survey in record time, which surprised everyone.

Here, which does not refer to time — it refers to the entire fact that the team completed the survey in record time. The relative clause comments on this fact.

Scientists discovered a previously unknown ecosystem at the vent site, which transformed our understanding of the conditions necessary for life.

Which refers to the entire discovery — not just to the ecosystem.

This construction is extremely common in academic and formal writing — it allows writers to chain propositions together in an elegant and economical way.

G. Relative Adverbs — Where, When, and Why

Relative adverbs introduce relative clauses that refer to places, times, and reasons.

Where

Where introduces a relative clause modifying a noun of place.

The site where the vents were first observed is now designated as a protected area. The laboratory where the samples are analysed is located in Southampton.

Where can be replaced by at/in which in formal writing.

The site at which the vents were first observed is now protected. (formal)

When

When introduces a relative clause modifying a noun of time.

The year when the vents were discovered was 1977. The decade when deep-sea exploration accelerated most rapidly was the 1990s.

When can be replaced by in which.

The year in which the vents were discovered was 1977. (formal)

Why

Why introduces a relative clause after the noun reason.

The reason why the species is found only at this site is not yet understood. Scientists do not yet know the reason why certain vent species are distributed so widely.

Why can be replaced by for which or simply omitted after reason.

The reason for which the species is found only here is unknown. (formal) The reason the species is found only here is unknown. (why omitted — informal)

3. Usage in Context

• Use a defining relative clause to identify which specific person or thing is meant — no commas, that possible for things.

The species that colonises the edges of active vent fields can tolerate temperatures that would be lethal to almost any other organism. The researchers who conducted the original survey returned to the site twenty years later.

• Use a non-defining relative clause to add supplementary information about an already-identified noun — always with commas, which for things, never that.

The hydrothermal vent community, which was first observed in 1977, remains one of the most studied ecosystems on Earth. Professor Chen, who has spent forty years studying vent ecosystems, was awarded the international prize for scientific achievement.

• Choose between defining and non-defining carefully — the punctuation changes the meaning.

The researchers who conducted the survey were exhausted. (defining — specifies which researchers; others may not be exhausted) The researchers, who had conducted the survey, were exhausted. (non-defining — all the researchers are meant; the survey detail is extra)

• Use whom — not who — when the relative pronoun is the object of the verb or follows a preposition in formal writing.

The researcher to whom the prize was awarded has since retired. (formal — whom after preposition) The scientist whom the committee selected led the expedition. (formal — whom as object)

• Use whose for both people and things when a possessive relationship is expressed.

The team whose findings transformed the field returned to the site a decade later. The vent ecosystem whose extraordinary diversity continues to astonish researchers was first mapped in the 1980s.

• Use preposition + which/whom in formal and academic writing — avoid stranded prepositions in formal prose.

The site at which the first vent community was observed is now a protected area. (formal) The site which the first vent community was observed at is now protected. (informal — acceptable in speech)

• Omit the relative pronoun only when it is the object of the relative clause in a defining clause — never in non-defining clauses and never when it is the subject.

The data (which) the team collected over three months filled several terabytes of storage. (object — omission possible) The species that colonised the site was previously unknown. (subject — omission not possible)

• Use which to refer to an entire preceding proposition — the sentential relative — to chain ideas economically.

The survey revealed more than two hundred previously undescribed species, which confirmed that the region was far more biodiverse than had been assumed. The funding was finally approved after a two-year review process, which allowed the expedition to depart on schedule.

• Use where as a relative adverb to avoid the awkward at which after nouns of place in semi-formal writing.

The laboratory where the samples are processed is equipped with the most advanced analytical instruments available. The site where the vents were first observed has been continuously monitored for nearly fifty years.

• In academic writing, use non-defining relative clauses to add qualifications, context, and additional evidence without breaking the flow of the argument.

The 1977 expedition, which was conducted using the research submersible Alvin, produced findings that permanently transformed our understanding of life on Earth. The regulatory framework, which has not been substantially updated since 1994, is widely regarded as inadequate for the current scale of deep-sea mining operations.

• Use the/a + noun + of which constructions as alternatives to whose in very formal writing when referring to things.

The ecosystem, the complexity of which continues to surprise researchers, was first surveyed in the early 1980s. The data set, the integrity of which had not been independently verified, was subsequently withdrawn from publication.

4. Common Errors and Corrections

5. Lesson Mastery

After completing this lesson, you should now be able to:

    ✅ Distinguish defining and non-defining relative clauses and punctuate them correctly

    ✅ Use the correct relative pronoun — who, whom, whose, which, that — in all contexts

    ✅ Understand when the relative pronoun can be omitted

    ✅ Form and use preposition + relative pronoun constructions in formal writing

    ✅ Recognise and correct common errors in relative clause formation and use