Learn French: Nous avons attrapé tous les moustiques qui volent dans la maison. - We caught all the mosquitoes that fly in the house.

We use qui when the word it replaces is the subject of the action that follows. In this sentence, the mosquitoes are the ones doing the flying (volent). If the mosquitoes were the object being acted upon (for example, the mosquitoes that we hate), you would use que instead.

Why is the verb volent in the plural -ent form?

The relative pronoun qui adopts the plural or singular form of the noun it replaces. Here, qui refers back to les moustiques, which is plural. Therefore, the verb voler (to fly) must be conjugated in the plural ils form: volent.

Do I pronounce the s at the end of tous in this sentence?

No, the s is silent here. When tous is placed before a noun to mean all the (as in tous les moustiques), it is pronounced like too. You generally only pronounce the s when tous stands alone as a pronoun meaning all of them.

Why does the tense change from past to present in the middle of the sentence?

The two verbs represent different timeframes. We caught (nous avons attrapé) is a completed action in the past. However, the descriptive part about the mosquitoes (qui volent dans la maison) is in the present tense because it describes a general, ongoing fact about what those mosquitoes do.