Relative pronouns let you combine two short sentences into one longer, more natural one, by turning the second sentence into a description of a noun in the first. Which pronoun you need depends entirely on the role that noun plays in the second sentence.
1. Qui: the subject of the clause
Use qui when the noun being described is the one doing the action in the next part of the sentence, it's immediately followed by a verb.
| J'ai un ami. Il habite à Paris. → J'ai un ami qui habite à Paris. | I have a friend who lives in Paris. |
2. Que: the object of the clause
Use que (qu' before a vowel) when the noun is receiving the action, it's followed by a subject and then a verb.
| Je lis un livre. Il est intéressant. → Le livre que je lis est intéressant. | The book that I'm reading is interesting. |
3. Où: place or time
| La ville où j'habite est belle. | The city where I live is beautiful. |
| Le jour où je suis né... | The day when I was born... |
4. Dont: replacing de + noun
The test: if the verb or expression in the second sentence needs de (parler de, avoir besoin de, le père de...), use dont instead of repeating de. C'est le livre dont je parle. (It's the book I'm talking about, from parler de ce livre.) La fille dont le père est médecin... (The girl whose father is a doctor, le père de la fille.)
5. Quick reference
| qui | subject, followed by a verb |
| que | object, followed by subject + verb |
| où | place or time |
| dont | replaces de + noun |