When do you use the subjunctive?

The subjunctive "mood" is usually used to express a will/order or doubt/possibility. There are also certain phrases which the subjunctive always follows.

For example:

Il est essentiel que tu fasses les devoirs  - It is essential that you do the homework

Je doute qu'il soit possible - I doubt that it is possible

Phrases that the subjunctive will always follow include:

Bien que - although

Afin que - in order that

A moins que  - unless

There are of course other examples but they all generally follow the above rules, so it is often starightforward to work out where to use the subjunctive.