Can someone please explain the Subjunctive to me!

The French subjonctif (subjunctive mood) can be very confusing for English speakers since its use has diminished in English, but is still in common use in Latin-based (or Romance) languages. The easiest way for English speakers to understand the French subjonctif is to first see cases in English.

It still exists in English, but it's only noticeable in a few cases because it seems a bit archaic (it appears a lot in Shakespeare, for example). Where it is still in common use, it's less noticeable because it's often identical to other forms. These English sentences are all in the subjunctive:

Be that as it may! (still in modern use!)

If I were you... (still in use)

Were I so kind! (modern: if only I was so kind)

Notice that all these cases express a hypothetical situation - something that hasn't happened but is desired or hypothesised - which is the hallmark of the subjunctive mood. In English, it often follows expressions like "I wish..." or "If X were [true]"

In English as in French, the subjunctive most often follows the word 'that' (que).

Alan insists that people be there on time.

Would that I were in charge! (modern: if only I was in charge)

I would like that he do it. (modern: I'd like him to do it)

French, of course, has a whole set of conjugations especially for the subjunctive mood, which makes can learning it more difficult, but like anything it's just a matter of practice.

Learning when to use Le Subjonctif Présent is greatly simplified by noticing that it generally follows 'que' (Il faut que, Il est necessaire que, bien que, pour que, etc.) and always expresses a hypothetical situation.