When talking about two future events, English often uses a present or present perfect tense after words like "when" or "as soon as" (e.g., "When we have finished, we will understand").
In French, if both actions take place in the future, both verbs take a future tense.
To show that one action will be completely finished before the second begins, use the futur antérieur for the first action, and the futur simple for the action that follows.
Using quand (when)
The verb immediately following quand takes the futur antérieur, while the result in the main clause takes the futur simple.
| When we have finished the census, we will understand the demography. | Quand nous aurons fini le recensement, nous comprendrons la démographie. |
| When the urban population has aged, it will decrease. | Quand la population urbaine aura vieilli, elle diminuera. |
Using dès que (as soon as)
Similarly, dès que emphasizes that the second action happens immediately after the first is completed. (Remember that que contracts to qu' before a vowel).
| As soon as they have analyzed the statistics, we will understand the demography. | Dès qu'ils auront analysé les statistiques, nous comprendrons la démographie. |
| As soon as the people have migrated, the urban density will increase. | Dès que les gens auront migré, la densité urbaine augmentera. |
Changing the clause order
You can swap the order of the clauses without changing the meaning. The temporal conjunction (quand or dès que) and the futur antérieur always stick together, even if they move to the end of the sentence.
| Several families will migrate when they have sold their farm. | Plusieurs familles migreront quand elles auront vendu leur ferme. |
| Each citizen will receive a message as soon as the government has organized the census. | Chaque citoyen recevra un message dès que le gouvernement aura organisé le recensement. |
