When do I need to use the subjunctive?

The subjunctive is one of the hardest grammar points that you need to tackle in French A level. Any signs that you understand it and can use it confidently will impress the examiner hugely. 

The subjunctive is a mood, in other words it's not a tense like the passé composé but it can be in the passé composé. 

Here's a handy list of times you need to use the subjunctive. 

1. After certain prepositions, such as: 
bien que...
quoique...

2. After verbal constructions, such as: 
avoir peur que...
avoir désir que...

3. After a superlative, such as:
c'est la plus jolie fille que j'aie vue. 

4. Impersonal expressions, such as: 
il faut que...

5. After a change of subject, expressing wish/want:
il était content que je l'aie fait. 

6: In short, you use the subjunctive in expressions of...
Will, orders, advice
Desires, emotions
Doubt, possibility, opinion

Finally, the best way to learn when to use the subjunctive is fimiliarity with written French. I would recommend reading articles on Euronews, as you can translate them into English if you are stuck as to why there is a subjunctive.