Learn French: Nous doutons que cette conséquence soit grave. - We doubt that this consequence will be serious.

The verb douter (to doubt) expresses uncertainty. In French, expressing doubt or improbability triggers the subjunctive, just like impersonal phrases such as il est peu probable que (it is unlikely that). You are showing that the consequence being serious is not a guaranteed fact.

If the sentence were negative (Nous ne doutons pas que), would it still take the subjunctive?

Actually, no! If you say Nous ne doutons pas que, you are saying we do not doubt that, which means you are certain. Because you are expressing certainty rather than doubt or improbability, you switch to the indicative: Nous ne doutons pas que cette conséquence est grave.

The English translation says will be serious, but soit is the present subjunctive. Why is it not in the future tense?

French does not have a future subjunctive tense. Whenever you need to express a future action that requires the subjunctive, you simply use the present subjunctive. The context makes it clear that the consequence lies in the future.

Can I use sérieuse instead of grave for serious in this sentence?

Grave is the better choice here. In French, grave means serious in the sense of severe, dangerous, or having bad consequences. Sérieux or sérieuse is generally used to describe someone who is earnest and hardworking, or a situation that is not a joke.