Pronouns must agree with the nouns they talk about. When talking about two nouns joined by and you must use a plural pronoun.
- Mary and David went to college, where they both graduated.
(two nouns/Mary and David – plural pronoun/both)
When two nouns are joined by either… or or neither… nor the pronoun takes the nearest noun.
- Neither the dog nor the cats could find their food.
(nearest noun/cats – plural pronoun/their)
