The subjunctive has always been a slightly misunderstood part of French grammar. It differs from tenses such as the present and future, as it is described as a 'mood' and is used to express doubt, uncertainity, possibility...Basically, it is used to express subjective actions and ideas.

As well as this, the subjunctive is used in many clauses with que and qui. For example; je ne pense pas que + subjunctive, il est necessaire que + subjunctive, il faut que + subjunctive.

An imperfect subjunctive does exist in French, but it is not as widely used as it is in other languages such as Spanish or Italian.