U12 - Present Perfect
Theory:
http://www.helpforenglish.cz/article/2008101805-predpritomny-cas-1-uvod-a-tvoreni
http://www.helpforenglish.cz/article/2006030602-predpritomny-cas
http://www.englisch-hilfen.de/en/grammar/present_perfect_form.htm
Do exercises:
http://www.helpforenglish.cz/article/2008033101-test-already-yet-still
http://www.englisch-hilfen.de/en/exercises/tenses/present_perfect_statements.htm
http://www.englisch-hilfen.de/en/exercises/tenses/present_perfect_negation.htm
http://www.englisch-hilfen.de/en/exercises/questions/present_perfect.htm
http://www.ego4u.com/en/cram-up/tests/present-perfect-simple-1
http://www.ego4u.com/en/cram-up/tests/present-perfect-simple-2
http://www.ego4u.com/en/cram-up/grammar/simpas-preper/exercises?ex07
http://testy.web2001.cz/A2_present_perfectS_01.html