Can you help me understand how to use the subjunctive in French?

There are some expressions such as "bien que" that always take the subjunctive which I'm sure you've heard about (if not then I can direct you to some useful lists, because they're really important to know). As for why it is used though, it is good to remember that the subjunctive is all about possibility. For example, when you say "Il faut que je parte" (I should leave), you use the subjunctive because while you may be obliged to leave, there's still the possibility that you'll stay. This should help you memorise the expressions that take the subjunctive, since you'll notice that the majority of them are concerned with possibility. Forming the subjunctive is normally pretty straightforward. In general you take the 'ils/elles' form of the verb (e.g. "marchent"), remove the '-ent' ending and replace it with one of '-e', '-es', '-e', '-ions', '-iez' or '-ent' depending on the subject of the verb. Sometimes this form will look the same as the indicative (normal) form, so remembering that it often occurs after "que" (as well as learning the constructions it's used with) will help you to notice it. There are some irregular forms such as "être", "avoir", "aller" which have to be learnt, and I can show you how they are declined if you'd like.