A brilliant website to help you understand and work the different aspects of grammar is in face a German-made website!
It is called English Grammar Online
You can read the explanations and try the exercises online!
For example if you want to work on the different tenses you have a special table here with all the details.
Afterwards you can choose from a great selection of grammar exercises that are corrected automatically at the end. If you need to be reminded of the rules you will find them just next to the exercises!