GENITIVE CASE

http://www.englisch-hilfen.de/en/exercises/nouns_articles/apostrophe_s.htm

http://www.autoenglish.org/basic/possessivepal/possessive.html

http://suz.digitaleschulebayern.de/english/grammar/saxong3.htm

http://resources.hwb.wales.gov.uk/VTC/apostrophes_possess/eng/Introduction/act3.swf

http://first-english.org/english_learning/english_beginners/s_genitive_exercises/01_s_genitive_exercises.htm