Present perfect II

A very simple explanation of yet, already, still, just. Follow the different screens and you'll understand their use.
http://www.iesromerovargas.net/~ingles/english/eso4/adverb1.htm

Explanation + 3 exercises
http://web2.uvcs.uvic.ca/elc/studyzone/330/grammar/upperf.htm

Here you have some more.
http://perso.wanadoo.es/autoenglish/gr.presper.i.htm

http://perso.wanadoo.es/autoenglish/gr.since.i.htm

5 quick exercises about for, since, already, just, never

http://english.baladre.org/sedaviwebfront/grammarindex.htm#yet

Present Perfect

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All the past tensens explained in Spanish, in case you still have trouble understanding tham.

http://www.mansioningles.com/gram40.htm

Exercises: Simple past- present perfect

http://www.englishpage.com/verbpage/verbs5.htm

http://www.englishpage.com/verbpage/verbs6.htm