This is a fantastic French grammar resource - their books are essential for higher level learning and this website is great for A-level or more advanced students, as it was designed for the French by the French! A great way to grasp concepts and learn in the language rather than translating into English.
The website is here: http://bescherelle.com/lemploi-du-subjonctif-imparfait
See down the side for further explanations of this tricky bit of grammar...