Who vs. Whom and That vs. Which: Rules and Examples

Overview

The choice between who and whom, and the choice between that and which, are two distinctions that trip up even advanced writers. Both pairs follow clear rules, but applying those rules correctly requires some grammatical awareness.

Who and whom both refer to people but occupy different grammatical positions within a clause. Who is a subject pronoun; whom is an object pronoun. The distinction mirrors the difference between he and him, or she and her.

That and which both introduce relative clauses but do different grammatical work. That introduces restrictive clauses, which define the noun they modify. Which introduces non-restrictive clauses, which add supplementary information. A comma almost always separates a non-restrictive clause from the noun it follows.

Who vs. Whom

The Subject and Object Distinction

Who is a subject pronoun — it performs the action of the verb in its clause. Whom is an object pronoun — it receives the action of the verb or follows a preposition.

The most practical method for deciding between them is the substitution test. Replace the word in question with he or she for subject uses, and with him or her for object uses. If he or she fits, use who. If him or her fits, use whom.

Who and Whom in Questions

In direct questions, who asks about the subject and whom asks about the object. Questions beginning with whom are more formal and less common in conversational English but are standard in written and professional contexts.

When a preposition governs the pronoun, whom is always required. Ending a question with a preposition and using who is acceptable in informal speech but is considered non-standard in formal writing.

Who and Whom in Relative Clauses

In relative clauses, the choice depends on the role the pronoun plays within that clause, not in the main clause.

That vs. Which

Restrictive and Non-Restrictive Clauses

The distinction between that and which rests on the type of relative clause being introduced.

A restrictive clause defines or limits the noun it modifies. It provides information essential to identifying which specific thing is being discussed. Removing it would change the meaning of the sentence. Restrictive clauses are introduced by that and are not set off by commas.

A non-restrictive clause adds supplementary information about a noun that is already fully identified. Removing it does not change the core meaning of the sentence. Non-restrictive clauses are introduced by which and are always set off by commas.

The comma test works reliably in most cases: if a comma precedes the relative clause, use which. If there is no comma, use that.

That in Restrictive Clauses

That introduces a clause essential to the meaning of the sentence. It cannot be removed without leaving the noun underdefined.

Which in Non-Restrictive Clauses

Which introduces a clause that adds information but does not define or limit the noun. The noun is already fully identified before the clause begins, and the clause can be removed without affecting the core meaning.

British and American Usage

In American English, the distinction between that and which is observed more strictly in formal writing. In British English, which is sometimes used in restrictive clauses without a comma, and this is considered acceptable in many contexts. For formal, academic, or international writing, maintaining the distinction produces the clearest and most universally accepted prose.

All Four Words Compared

Common Mistakes to Avoid

Mistake 1: Using Who Instead of Whom After a Preposition

Prepositions always govern object pronouns. Any pronoun that follows a preposition must be in the object form.

Mistake 2: Using Whom Instead of Who as the Subject of a Clause

Hypercorrection is common with whom. Some writers use it in all formal contexts, including positions where the pronoun is actually the subject.

Mistake 3: Using Which Instead of That in Restrictive Clauses

Using which without a comma in a restrictive clause blurs the essential and supplementary distinction. In formal writing, restrictive clauses require that.

Mistake 4: Using That in Non-Restrictive Clauses

That cannot introduce a non-restrictive clause. When a comma precedes the relative clause, which is the only standard option.

Mistake 5: Omitting the Comma Before Which in Non-Restrictive Clauses

The comma is not optional in non-restrictive clauses. Omitting it changes the clause from non-restrictive to restrictive in interpretation, altering the meaning of the sentence.

Where the intended meaning is non-restrictive, the comma is required: The building, which opened last year, is already at capacity.

Mistake 6: Using That to Refer to People in Formal Writing

While that can technically refer to people in restrictive clauses in informal usage, who is strongly preferred in formal and academic writing.

Practice Exercises

Exercise 1: Who or Whom

Choose the correct word to complete each sentence.

1. _____ drafted the original version of the agreement?
2. The consultant _____ the board hired had extensive industry experience.
3. To _____ should I address the letter of complaint?
4. The applicant _____ submitted the strongest portfolio was offered the position.
5. _____ were you speaking with when I called earlier?

Exercise 2: That or Which

Choose the correct word and add a comma if necessary.

1. The only clause _____ needs revision is the liability section.
2. The liability clause _____ was added last year has since been challenged in court.
3. Any employee _____ works more than forty hours per week is entitled to overtime pay.
4. The main conference room _____ holds up to sixty people is currently being renovated.
5. The method _____ produces the most consistent results should be adopted as standard.

Exercise 3: Identify and Correct the Error

Each sentence contains one error. Rewrite the sentence correctly.

1. The manager who the team most respected announced her resignation yesterday.
2. The document which must be signed before the deadline is on your desk.
3. She is the only person whom understands the full scope of the project.
4. The funding, that was approved in March, has already been allocated.
5. To who did you send the completed application?

Summary

Both distinctions are governed by consistent rules. The substitution test and the comma test make them reliable to apply in practice.