A pronoun must agree with the noun it talks about.
- John has brought his book.
(noun/John singular man – singular male pronoun/his) - My sisters like to put up their Christmas tree early.
(noun/my sisters plural – plural pronoun/their) - Each of the girls gave her own version of the story.
(Each is singular – girl/her) - Gary and I would like to see a film this weekend, but we can’t agree which film.
(Gary and I/we)