The following sites should help you to develop your grammar skills

http://www.resnet.wm.edu/~hkcrai/exercises.htm   

http://www.colby.edu/personal/bknelson/grammar.html 

http://www.trinity.edu/mstroud/grammar/index.html  

http://www.colby.edu/personal/b/bknelson/grammar.html  

http://www.colby.edu/personal/b/bknelson/