I highly recommend this website if you are interested in learning or gaining a stronger understanding of the English Language. All the lessons are free and come with exercises to practice what you've learned.
I'd like to note that this site has an option for teachers to use it as an Online Class (You can't do the quizzes or test unless you are part of a class) and I am not 100% sure how that part works.
For me personally, I've always struggled on Grammar. This Website has helped me greatly.
English Grammar 101
A Free Web-based program for learning English Grammar.