Learn Portuguese (Portugal): Supondo que eles cancelem o voo, nós remarcamos a viagem. - Supposing that they cancel the flight, we reschedule the trip.

The phrase supondo que (supposing that) introduces a hypothetical scenario. Whenever you frame a sentence with a condition that isn't established as a fact, Portuguese requires the subjunctive mood. That is why we use the present subjunctive cancelem rather than the indicative cancelam.

Why does the main clause use the present tense remarcamos instead of the future tense?

It is very common in Portuguese to use the present indicative to state a firm plan or a guaranteed outcome in the future. You could use the future tense remarcaremos (we will reschedule), but remarcamos sounds more decisive and natural for everyday spoken Portuguese.

Why do we use the gerund supondo here? I thought European Portuguese avoided the gerund.

While European Portuguese strongly prefers a plus the infinitive for continuous actions, the gerund is still standard for adverbial phrases and certain conjunctions. Set phrases like supondo que or imaginando que always use the gerund to establish the condition.

Why doesn't voo have an accent mark?