Que and Qui are relative pronouns used to introduce relative clauses the way that Which, That and Who/Whom do in English, but their respective grammatical usages follow different rules.

Like Which and That, Que and Qui have the same meaning, but whilst Which and That are more or less grammatically interchangeable in English (except if the relative clause is independent, in which case Which must be used) Que and Qui are not in French.

- La lettre qui est sur la table.
- The letter that/which is on the table.

Here Qui is referring to letter as the subject of the verb to be.

- L’événement qui a changé le monde.
- The event that/which changed the world.

Qui refers to event, which is the thing doing the action of changing.

Que

Que, on the other hand, is only used if it refers back to the object of a sentence:

- C’est la lettre que j’écris.
- It’s the letter that/which I am writing.

Que refers to letter, which is being written (and is therefore the receipt point of the verb).

- Je veux la pomme que tu as achetée.
- I want the apple that/which you bought.

Here Que refers to apple, which is the object of the verb to buy.

Elided ‘e’ of Que

A noteworthy peculiarity is that with Que, the ‘e’ is elided (omitted/contracted) and replaced by an apostrophe when the first letter of the following word is a vowel. This isn’t so with Qui:

- La tarte qu’elle a sortie du four.
- The tart that/which she took out of the oven.

- La tarte qui est cuite.
- The tart that/which is cooked.

Qui/Que versus Who/Whom

The other significant difference in the usage of Que and Qui is that, unlike English, French does not distinguish between animate and inanimate objects (people and things) in the use of relative pronouns.

Whilst Que and Qui can be used to refer to people, in English Who or Whom are used exclusively for this:

- L’homme qui parlait.
- The man who was speaking.

- Les enfants auxquels j’enseigne.
- The children whom I teach.

(‘Auxquels’ is a plural variation of ‘Que’ in French. The singular form of ‘Auxquels’ is ‘à qui’ — ‘to whom’. Observe the root ‘que’ in ‘auxquels’.)

One cannot say:

- The man which/that was speaking.
- The children which/that I teach.

Who/Qui versus Whom/Que

However, whilst English does not distinguish between the object or subject of the sentence when using That and Which, it does when referring to people. Who, of course, is used when referring to the subject, and Whom when referring to the object of the sentence:

- This is the man who entered the room.
- C’est l’homme qui est entré dans la sale.

Who here refers to man, and man performs the action of entering (and is hence the subject), whereas:

- This is the man whom I saw.
- C’est l’homme que j’ai vu.

Who here refers to man, and he is the subject (receiver) of the verb to see.

Note: Qui as an interrogative pronoun also means Who:

- C’est qui ? (or, more correctly in French: “Qui est-ce?”)
- Who is it?

And lastly, unlike English where this can occasionally be done, the relative pronoun in French can never be omitted:

- C’est la pomme je veux.

doesn’t make any sense in French (the correct structure is “C’est la pomme que je veux”), whereas:

- This is the apple I want.

is perfectly acceptable in English.

M. Birch, M.A. Oxford