Simple present X present Continuous
http://www.englishpage.com/verbpage/verbs2.htm

Present Continuous X Present simple
http://www.englishpage.com/verbpage/verbs1.htm

Present Continuous
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/pcont1.htm

Was - Were 1
http://a4esl.org/q/h/lb/was.html

Was - Were
http://a4esl.org/q/h/lb/waswere.html

Have - Has
http://a4esl.org/q/h/lb/have.html

Is - Are - Has
http://a4esl.org/q/h/0001/sb-present.html

Present Simple
http://a4esl.org/q/h/lb/psv.html

Present Simple (negative verbs)
http://a4esl.org/q/h/lb/psnv.html

Subject + Verb ( agreement)
http://a4esl.org/q/j/kf/mc-svae.html

Short Answers (to be, can)
http://a4esl.org/q/h/9801/cg-shortans.html