1) What is a relative clause?

A relative clause adds information about a noun (person, thing, idea). It is introduced by a relative pronoun (who, whom, whose, which, that) or a relative adverb (where, when, why).

• Defining (= identifying) The student who sits by the window won the prize. (tells us which student)
• Non-defining (= extra) Maria, who sits by the window, won the prize. (extra detail about Maria)

2) Core contrast & comma rule

• The houses that face the park are expensive.
• My house, which faces the park, gets a lot of light.

If removing the clause changes who/what we mean, it’s defining. If removal leaves reference clear, it’s non-defining.

3) Relative pronouns: choices & register

Spoken, informal English often prefers that in defining clauses. Avoid that after a comma or after a preposition placed before the pronoun.

4) Relative adverbs: time, place, reason

• when 2005 was the year when we met.
• where This is the café where they first talked.
• why I’ll never forget the reason why he left. (Many writers drop why after reason: the reason he left.)

More formal equivalents use a preposition + which: the year in which…, the café at which…

5) Prepositions in relative clauses

Preposition + which/whom (formal; written)

• She finally bought the violin for which she had saved for years.
• He was a mentor to whom many young lawyers turned.

… + preposition (at the end) (neutral/informal)

• She finally bought the violin (which/that) she had saved for years for.

6) Omission (Ø) of the relative pronoun

• Allowed The book Ø I told you about is out of print. (object)
• Allowed The colleague Ø you were talking to called back. (object of preposition; preposition stranded)
• Not allowed *The engineer Ø fixed the server is here. (subject)
• Not allowed Omission is never used in non-defining clauses.

7) which/that vs what

• Referring back which/that point back to a noun or to a whole clause.
  I kept the postcard which/that she sent me.
  He offered to help us, which was very kind. (refers to the whole idea)
• Independent what means “the thing(s) that” and does not follow a noun.
  I didn’t like what he said. / What you need is a break.
• ✱ Don’t use what as a relative pronoun after a noun: *the postcard what…

8) Quantifier + of which/whom (non-defining only)

• The engineers, many of whom work remotely, meet monthly.
• The shelters, none of which were full, stayed open all night.
• The library owns rare journals, several of which date from the 19th century.
• We joined a hiking group, most of whose members are families.

Typical quantifiers: some, any, none, all, both, several, many, most, few, each, either, neither.

9) Special tendencies with that

• Superlatives/only Defining clauses after the first/only/best/next often use that: She’s the only person that understands.
• Indefinite pronouns After everything, nothing, something, anything use that: Tell me everything that happened.
• No commas that does not occur in non-defining clauses.

10) Clause-reference which (non-defining)

which can refer to a whole idea in the previous clause and must be set off by commas:

• The flight was delayed, which meant we missed the connection.
• He ignored the safety rules, which was irresponsible.

11) Quick reference

12) Editing tests you can use

• Which one? test If the clause answers “Which one?”, it’s defining.
• By-the-way test If the clause feels like an aside, use commas → non-defining.
• Comma ban Never mix commas with that; switch to who/which.