Excellent guide to English grammar. It focuses on verbs and their times. Contains many tests for self-examination and answers to them. Ideal for additional classes on your own.
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http://usafiles.net/1VRr/Test_It_Fix_It_English_Grammar_Intermediate.pdf
http://usafiles.net/1VRs/Test_It_Fix_It_English_Grammar_Pre_Intermediate.pdf