Expressions of probability
When discussing scientific predictions or future events, we often state how likely they are to happen. In French, il est probable que (it is probable that) expresses a high degree of certainty. Because the event is viewed as a near-fact, it is followed by the indicative mood, just like a normal statement.
| It is probable that the glacier will melt. | Il est probable que le glacier fondra. |
| It is probable that the volcano's eruption will happen tomorrow. | Il est probable que l'éruption du volcan aura lieu demain. |
Expressions of improbability
Conversely, when an event is unlikely, certainty is lost. Expressions of doubt like il est peu probable que (it is unlikely that) change the reality of the statement. Because of this uncertainty, they require the subjunctive mood.
Notice how the verbs in the second part of these sentences shift into the subjunctive (like soit for être or détruise for détruire):
| It is unlikely that the asteroid's trajectory is dangerous. | Il est peu probable que la trajectoire de l'astéroïde soit dangereuse. |
| It is unlikely that the earthquake will destroy the city. | Il est peu probable que le séisme détruise la ville. |
Other levels of certainty
The rule of thumb is simple: if it's probable, use the indicative. If it's merely possible or completely improbable, the doubt triggers the subjunctive.
Expressions like il est possible que (it is possible that) and il est improbable que (it is improbable that) both take the subjunctive.
| It is possible that this eruption will happen today. | Il est possible que cette éruption ait lieu aujourd'hui. |
| It is improbable that the earthquake will have serious consequences. | Il est improbable que le séisme ait des conséquences graves. |
