Here you find some links that might help you to understand the differences between English tenses and you can practise them online if you like.

General

http://wwwedu.ge.ch/cptic/prospective/projets/anglais/exercises/welcome.html

http://www.englishdaily626.com

Irregular Verbs

http://www.aitech.ac.jp/~iteslj/quizzes/irv001-ck.html

Since / For

http://www.aitech.ac.jp/~iteslj/quizzes/js/ck/ch-forsince.html

Explanations

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/pres.htm - Forming the simple present

http://www.ucl.ac.uk/internet-grammar/verbs/xverb1.htm - On verbs

http://www.edict.com.hk/funcgrammar/tense/tense.htm - The meaning of tense (advanced)

Excercises

http://www.better-english.com/grammar.htm

http://www.edict.com.hk/vlc/verbs/verbsind.htm

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/pres1c.htm

http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/index.htm

http://iteslj.org/quizzes/lb/psnv.html

http://vlc.polyu.edu.hk/verbs/verbs4.htm

http://englishpage.com/verbpage/verbtenseintro.html