You can find different sites that can help you to understand the English grammar much better!

http://www.edufind.com/english/grammar/toc.cfm

http://www.autoenglish.org/tenses.htm

http://www.english-grammar-lessons.com/

http://mbonillo.xavierre.com/httpdocs/grammar/4eso.html

http://www.miguelmllop.com/practice/intermediate/verbtenses/verbtensesummary.htm

http://acacia.cnice.mecd.es/%7Eagip0002/auro/inicio.html

http://www.agendaweb.org/

http://a4esl.org/q/j/ck/fb-queswords.html

An excellent Canadian web site where you can find help if your level is Pre-intermediate, Intermediate or Upper-intermediate:

http://wwwedu.ge.ch/cptic/prospective/projets/anglais/page2.htm#grammar

Another Canadian web site from the University of Victoria with different levels and exercises:

http://web2.uvcs.uvic.ca/elc/studyzone/index.htm

Excellent web site where you can practice grammar:

http://www.english-grammar-lessons.com/pastsimple/menu.php

Online English course

http://www.1-language.com/englishcourse/index.htm

American English Grammar in  three levels

http://www2.cambridge.org/interchangearcade/main.do

Another interesting place to work with grammar

http://www.roadtogrammar.com/