You already met defining relative clauses in Lesson 2 — they give essential information that identifies exactly who or what we mean. Non-defining clauses are different: they add EXTRA information about someone or something we've already identified. Remove a non-defining clause, and the sentence still makes complete sense.
RULE
Defining clause: essential — tells us WHICH person/thing. No commas. Removing it changes the meaning.
Non-defining clause: extra — adds more detail about someone/something already clear. Commas required. Removing it loses detail, but the core meaning stays the same.
Test: if you can delete the clause and the sentence still identifies the same person/thing clearly, it's non-defining.
TIP
Say the sentence without the clause. If it still makes complete sense on its own, you probably need commas (non-defining). If it becomes vague or incomplete, no commas (defining).
EXAMPLES
Defining: The judge who noticed the similarities between languages was British. (tells us WHICH judge — essential)
Non-defining: Sir William Jones, who noticed the similarities between languages, was a British judge living in India. (we already know who he is — the clause just adds detail)
Defining: The language that Jones began to study was Sanskrit. (identifies which language)
Non-defining: Sanskrit, which Jones began to study in 1786, is a very old language of India. (Sanskrit is already named — extra fact)
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Quiz · choose one option per question
Is the relative clause defining or non-defining?
1.'The Kurgans, who lived in southern Russia, spoke an early form of Proto-Indo-European.' — the clause 'who lived in southern Russia' is...
2.'People who study Sanskrit often notice its similarity to Latin.' — the clause 'who study Sanskrit' is...
3.'King John, who lost Normandy in 1204, was forced to give up French lands.' — the clause 'who lost Normandy in 1204' is...
4.'The Normans who married English women often had children who spoke English.' — the first clause 'who married English women' is...
5.'The Black Death, which killed almost a third of the population, changed English society deeply.' — the clause 'which killed almost a third of the population' is...
6.'Words that entered English from French often described life at court.' — the clause 'that entered English from French' is...
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Theory · read before you start
Punctuation and 'that': Rules for Non-Defining Clauses
Non-defining clauses follow stricter rules than defining ones. Get these two rules right and you'll avoid the most common B1-level mistakes.
RULE
Always use commas around a non-defining clause — one before it, one after (unless it ends the sentence, then just one comma before).
Never use 'that' in a non-defining clause — only who / whom / which / whose. 'That' is for defining clauses only.
Use whose for possession in either type; use which for things, who for people, in both types too — the real difference is the commas and the 'that' rule, not the pronouns themselves.
TIP
If you catch yourself writing 'that' straight after a comma, stop — that combination never happens in correct English.
EXAMPLES
William the Conqueror, who became king in 1066, built castles across England.
Norman French, which remained the language of the court for two centuries, slowly lost importance.
The Anglo-Saxon Chronicle, which English monks continued writing until 1154, is a rare example of written English from this period.
Henry IV, whose reign began in 1399, was England's first English-speaking king since the Conquest.
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Gap fill · type your answer in each gap
Complete each non-defining clause with who, which, or whose. Remember: never 'that' here.
1.William, completed his conquest of England and Wales, was crowned in London.
2.Norman French, was the language of the governing classes, remained important for two centuries.
3.King John, lands in France were lost in 1204, weakened Norman ties to France.
4.The Cursor Mundi, was written by an English poet around 1300, defended the English language.
5.Henry IV, became king in 1399, was the first English-speaking king since 1066.
6.The Hundred Years War, lasted from 1337 to 1453, increased English national feeling.
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Theory · read before you start
Omitting the Pronoun: Defining Clauses Only
You learned in Lesson 2 that when the pronoun is the OBJECT of a defining clause, you can leave it out. That rule has one crucial limit: it never applies to non-defining clauses, no matter what role the pronoun plays.
RULE
Defining clause, pronoun = object → the pronoun is optional, you can omit it.
Defining clause, pronoun = subject → the pronoun is required, you cannot omit it.
Non-defining clause, any role → the pronoun is always required. Never omit it, even in object position.
TIP
Non-defining clauses are separated by commas precisely because they're a separate, extra piece of information — and you can't mark that separation at all if you delete the very word that starts the clause.
EXAMPLES
Defining, object, can omit: Norman French was the language (that) William's soldiers spoke.→ 'William's soldiers spoke' still makes sense alone.
Defining, subject, cannot omit: The king who lost Normandy was John.→ you cannot say 'The king lost Normandy was John.'
Non-defining, object, cannot omit: Latin, which the Church used for centuries, slowly gave way to English.→ you cannot say 'Latin, the Church used for centuries, slowly gave way to English.'
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Quiz · choose one option per question
Can the relative pronoun be omitted in this sentence?
1.'The language (that) the Normans brought to England was French.' — the pronoun here...
2.'French, which the Normans brought to England, became the language of the court.' — the pronoun here...
3.'The poet who wrote Cursor Mundi defended the English language.' — the pronoun here...
4.'Henry IV, who became king in 1399, was the first English-speaking king.' — the pronoun here...
5.'The lands (that) King John lost belonged to Normandy.' — the pronoun here...
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Correct the mistakes · type the correct word in the box
The highlighted word contains a mistake. Write the correct form.
1.
William the Conqueror that became king in 1066 built castles across England.
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2.
Norman French, was the language of the court, remained important for two centuries.
↳
3.
Latin, the Church used for centuries, slowly gave way to English.
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4.
King John who lost Normandy weakened English ties to France.
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5.
The Black Death that killed almost a third of the population changed English society.
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Sentence transform · type your rewritten sentence
Combine each pair of sentences into one, using a defining or non-defining relative clause as shown. Add commas where needed.
1.Henry IV became king in 1399. He was the first English-speaking king since 1066. (non-defining)
2.The lands belonged to Normandy. King John lost the lands. (defining, omit the pronoun)
3.The Cursor Mundi was written around 1300. It defended the English language. (non-defining)
4.The king lost Normandy in 1204. The king was John. (defining, subject, cannot omit)
5.The Hundred Years War lasted from 1337 to 1453. It increased English national feeling. (non-defining)