When looking out over the castle walls during a siege, you might find yourself guessing what the enemy is up to. In French, you can use future tenses to express probability or make assumptions about the present or the past.
Present Suppositions
To state that something is highly probable right now—often translated into English as "must be" or "is probably"—French commonly uses the future tense (le futur simple). This is known as the future of probability (le futur de probabilité).
| It's quiet outside, the enemy must be tired. | C'est calme dehors, l'ennemi sera fatigué. |
| The relief force must be late. | Le renfort sera en retard. |
| The sentry is no longer moving, she must be sleeping. | La sentinelle ne bouge plus, elle dormira. |
Past Suppositions
When you want to make an educated guess about something that has already happened, you will use the future perfect (le futur antérieur). In English, this is equivalent to saying someone "must have" done something.
You form the future perfect using the future tense of the auxiliary verb (avoir or être) followed by the past participle.
| The assault must have failed yesterday. | L'assaut aura raté hier. |
| The enemy must have destroyed the bridge. | L'ennemi aura détruit le pont. |
Remember that the standard rules for auxiliary verbs still apply. If the verb takes être—such as verbs of motion or pronominal verbs like se replier (to retreat)—the past participle must agree in gender and number with the subject.
| The troop must have retreated with the banner. | La troupe se sera repliée avec la bannière. |
| Where does this relief force come from? It must have come from the north. | D'où vient ce renfort ? Il sera venu du nord. |