Enlaces Past tense present simple

http://www.mansioningles.com/gram52.htm
http://www.learnamericanenglishonline.com/Red%20Level/R3%20Do%20and%20Did.html
esl.about.com/od/englishspanish/a/ingles_basico_verbos_irregulares.htm
www.perfect-english-grammar.com/verb-tenses.html