Definition of Terms:

Clause: A unit of phrase structure comprising a verb, and ordinarily also a subject.

Relative Clause: A clause that specifies or gives information about something or someone. These normally start out with a relative pronoun (who/whom/whose/which/that).

Subject: A word, phrase etc. that executes the action of the verb:

- She drove the car.
- The photograph is nice.

She and photograph are the subjects.

Object: A word or group of words that receive or are affected by the action of a verb:

- She ate the banana.

Banana is the object.

Pronoun: Any word used in place of a noun (you, he, she, I, etc. but also me, who, these etc.)

Relative Pronoun: A pronoun representing a noun in another (preceding) clause.

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

Like That and Which, Qui and Que have the same meaning, but while Which and That are just about grammatically exchangeable in English (except if the relative clause is independent, in which event Which must be used) Que and Qui are not in French.

Qui

Qui is exclusively used if it refers back to the direct subject of the sentence:

- La fleur qui est dans le vase.
- The flower that/which is in the vase.

Here Qui refers to flower as the subject of the verb to be.

- Le groupe qui nous a bien aidé.
- The group that/which helped us a lot.

Qui refers to group, which is the thing doing the action of helping.

Que

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

- C'est le livre que je suis en train de lire.
- It's the book that/which I am reading.

Que refers to book, which is being read (and is thus the receipt point of the verb).

- Je veux le tableau que tu as signé.
- I want the painting that/which you signed.

Here Que refers to painting, which is the direct object of to sign.

Elided 'e' of Que

A remarkable distinctive feature is that with Que, the 'e' is elided (omitted/shortened) and replaced by an apostrophe when the 1st letter of the next word is a vowel. This isn't so with Qui:

- Le flan qu'elle a retiré du four.
- The custard that/which she pulled out of the oven.

- La flan qui est cuit.
- The custard that/which is cooked.

Qui/Que versus Who/Whom

The additional significant difference in the utilization of Que and Qui is that, unlike English, French does not differentiate between living and nonliving objects (people and things) in the use of relative pronouns.

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

- La femme qui courait.
- The woman who was running.

- Les parents auxquels je m'adresse.
- The parents whom I talk to.

('Auxquels' is a plural form of 'Que' in French. The singular of 'Auxquels' is 'à qui' - 'to whom'. Observe the root 'que' in 'auxquels'.)

One cannot state:

- The woman which/that was running.
- The parents which/that I talk to.

Who/Qui versus Whom/Que

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

- This is the child who took the cookies.
- C'est l'enfant qui a pris les gâteaux.

Who here refers to child, and child performs the action of taking (and is therefore the subject), whereas:

- This is the person whom I saw.
- C'est la personne que j'ai vu.

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

Note: Qui as an interrogative pronoun also signifies Who:

- C'est qui ? (or, more correctly in French: "Qui est-ce?")
- Who is it?

And finally, unlike English where this can from time to time be done, the relative pronoun in French can never be left off:

- C'est la boison j'aime.

doesn't make any sense in French (the correct structure is "C'est la boison que j'aime"), whereas:

- That is the drink I like.

is absolutely satisfactory in English.

In general I always advise to use use a good grammar and dictionaries in case of difficulty with a language. But if still in doubt when translating or writing, ask a native speaking person. For important business project be sure to use existing translation services that will help you in your quest for foreign customers satisfaction.