The French subjunctive mood is one of the most difficult concepts for English speakers to learn. While English has a subjunctive mood, its grammatical formulations aren't quite as different from the indicative tenses.

You use the subjunctive for various reasons:

cases of desire
Je veux qu'ils viennent.
I want that they come.

to express doubt
Je doute que tu aille.
I doubt that you go.

to make impersonal statements
C'est bien qu'il lise tant.
It's good that he studies so much.

to talk about impeding events
Quand tu ailles, envoie mes salutations.
When you go, say "hi" for me.

French also has a past subjunctive that describes past events in the same way as above.