A CLEAR REFFERENCE BOOK FOR YOUR GRAMMAR

This is the perfect book to have as a reference book whenever you come across (encuentras) something that makes you doubt or that is hard to understand. Then come to this book, look at the index, and the asnswer will be probably somewhere inside.

http://library.aceondo.net/ebooks/English_Language/how_english_works_a_grammar_practice_book_oxford_1997.pdf