The Subjunctive is a **mood**, not a tense. It's used for subjective things: desires, doubts, emotions, and necessities. It's the mark of a true speaker.
Take the Ils form of the present tense, drop the -ent, and add Subjunctive endings.
Il faut que...
It is necessary that...
Je veux que...
I want that...
Je suis content que...
I am happy that...
Je doute que...
I doubt that...