Accuracy is only achieved by intensive practice.
Check out the websites below if you need to practice a particular grammar area:
https://owl.english.purdue.edu/exercises/2/22/
http://www.rong-chang.com/ex/contents.htm
A great website for more research on English grammar:
http://www.edufind.com/english-grammar/english-grammar-guide/
The University of Victoria (Canada), has developed a wonderful site which helps students with grammatical difficulties:
http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/index.htm