Defining relative clauses: who/which/that

A defining relative clause tells us which person or thing the speaker means — without it, the sentence loses its meaning. We use 'who' (or 'that') for people and 'which' (or 'that') for things, animals, and groups. There is NO comma before the clause because it is essential information: 'The boy who sits next to me is my friend' tells us exactly which boy. The relative pronoun can be the subject of the clause ('The dog that bit me was large') or the object ('The book that I read was interesting'). When the pronoun is the object, it can be left out entirely: 'The book I read was interesting.' 'Whose' shows possession ('the girl whose bag is red') and 'where' replaces a prepositional phrase of place — but these are introduced here only briefly. For example: 'The teacher who teaches us maths is very patient' — 'who teaches us maths' defines exactly which teacher, making 'who' compulsory here.