Common Errors With Relative Pronouns
Incorrect: Alice, that does my hair, has moved to another hairdresser’s.
Correct: Alice, who does my hair, has moved to another hairdresser’s.
The relative pronoun that can refer to both people and things, but it is not used in non-identifying relative clauses.
Note that non-identifying relative clauses simply give additional information. They can be easily left out. Identifying relative clauses, on the other hand, cannot be easily left out.
Incorrect: I poured him a glass of coffee, he drank at once.
Correct: I poured him a glass of coffee, which he drank at once.
Incorrect: She went to work with my cousin, she later married.
Correct: She went to work with my cousin, whom she later married.
The relative pronoun cannot be left out in non-identifying relative clauses.
Incorrect: This is the house what Jack built. Correct: This is the house that Jack built.
Incorrect: The only thing what keeps me awake is coffee. Correct: The only thing that keeps me awake is coffee.
What means that which or the things which. It cannot be used as an ordinary relative pronoun after a noun or pronoun.
Incorrect: That cannot be cured must be endured.
Correct: What cannot be cured must be endured. (= That which cannot be cured must be endured.)