The best thing you can do is read, read, read (I imagine you do this already) to increase your language exposure. This will help you to know "what just sounds right" and is honestly the way native speakers learned the language. Reading in conjunction with a grammar book should put you on the right path.
There aren't any exercises here but these websites might be able to help you with your writing:
Guide to Grammar and Writing
The Purdue OWL: General Writing
Student or Learner