How to Choose
'Who', 'whom', and 'that' are all relative pronouns. Two of them are used as a subject and one of them is the object. If you want to which is which, read this!
That vs. Who or Whom
What Do They Have in Common?
These three are all 'Relative pronouns' that introduce relative clauses.
A relative clause is a kind of dependent clause. It has a subject and a verb, but the meaning is not complete, and it cannot be a whole sentence.
Common Relative Pronouns
The most common relative pronouns in English are:
But Are They Interchangeable?
Depending on the type of relative clause, each one of them has its own uses. But as you will see later in this article, they can be interchangeable in some cases, and not in some. Here, we will focus on three relative pronouns: 'who', 'whom', and 'that'.
'Who' is a relative pronoun used as a subject to refer to people. 'That' is a relative pronoun used as a subject and object for things or groups. When used as an object, 'who' becomes 'whom.'
Whom is a relative pronoun that is used as the object of a relative clause.
Whom Is Not Very Common
The use of 'whom' is not very common in spoken English. Normally, people use 'who' both as subject and object pronoun of relative clauses.
Whom Is Very Formal
It is also considered very formal to use 'whom' after prepositions (as the object of prepositions):
In everyday English, it is more natural to use 'who' instead of 'whom' at the beginning of a question, then put the preposition at the end of the sentence.
In Essential Relative Clauses, Are They Interchangeable?
In essential relative clauses you can either use 'who' or 'that' or 'whom', or leave out the pronoun completely.
In Non-Essential Relative Clauses, Are They Interchangeable?
In non-essential relative clauses, you can use 'who' or 'whom', but you cannot use 'that'. Also you cannot leave out the pronoun.
The relative pronoun 'that' is very flexible and you can use it for people or things. Both these sentences are correct: