Learn French: Je doute que ce projet soit rentable. - I doubt that this project is profitable.

Soit is the subjunctive form of être (to be). We use the subjunctive here because the phrase Je doute que expresses uncertainty. In French, expressing doubt about whether an action or state is true requires the subjunctive mood in the clause that follows.

Would I still use the subjunctive if the sentence started with Je pense que?

No. Verbs like penser (to think) and croire (to believe) express a stated opinion when used in the affirmative. You would use the normal indicative mood: Je pense que ce projet est rentable. However, if you make them negative, like Je ne pense pas que, they express doubt and then they do trigger the subjunctive.

Is the word rentable related to the English word for paying rent?

It looks similar, but it actually comes from the French word la rente, which means revenue or income. When a project or business is rentable, it means it generates a good financial return, so it translates to profitable or cost-effective.

How would I say that I doubt that I MYSELF am profitable? Do I still use que?