U1 - U6 Revision
https://elt.oup.com/student/solutions/preint/grammar/grammar_01_012e?cc=cz&selLanguage=cs
http://anglictina.chrustova.eu/cviceni/present-simple-vs-continuous.php
http://anglictina.chrustova.eu/cviceni/present-simple-vs-continuous.php
http://www.focus.olsztyn.pl/en-the-simple-past-tense-exercises.html#.WvLeL4iFMdU
http://www.focus.olsztyn.pl/en-time-prepositions-in-at-exercise.html#.WvLeeIiFMdU
http://www.focus.olsztyn.pl/en-exercises-prepositions-time-test.html#.WvLeo4iFMdU