During vs. for

Explanation:

http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv170.shtml

 

http://www.youtube.com/watch?v=OXEKn9JF9II

 

Exercises:

http://www.tolearnenglish.com/exercises/exercise-english-2/exercise-english-44345.php

 

http://www.english-test.net/esl/learn/english/grammar/ei094/esl-test.php