Hello, 

The best advice I can give you to reduce grammar mistakes in your writings is to have a look at the different written tasks you are doing during the course, make a register of grammar mistakes made and identify the grammar involved and revise it. 

For those ones who need to revise a more basic grammar, here you have a series of useful websites:

Perfect English Grammar

English Grammar Today 

British Council -Grammar and Vocabulary 

Using English

English Club – Grammar