The following link will lead you to a website to learn English online for free. It is plenty of exercises dealing with grammar issues which you may have difficulties with, such as modal verbs, pronouns, adverbs, prepositions, etc with simple and clear explanations and the answer keys. You can choose from different levels and types of exercises, there are also video lessons and much more.

I encourage you to visit this useful site and do as many exercises as possible!

http://www.autoenglish.org

http://www.autoenglish.org/tenses.htm (click here to go directly to verb tenses: modals, conditionals…)

http://www.autoenglish.org/freeexercises.htm (click here to go directly to the grammar exercises)

Good luck!