Question

does anyone know of website where they teach grammar for free and give yopu interactive tests?

Answer 1

http://grammar.ccc.commnet.edu/grammar/index2.htm

Here's one. Choose the grammar or usage element you need to study, and at the bottom of each webpage are exercises and quizzes. For example, see this one on comma usage:
http://grammar.ccc.commnet.edu/grammar/commas.htm