If you want to be.....

a perfect friend (You SHOULD leave this bastard, honey.)
a perfect doctor (You MUST stay in bed for two weeks.)
a perfect detective (It CAN'T be Paul- he's too tall.)
or just a perfect English user, you NEED TO know modal verbs:)
http://www3.telus.net/linguisticsissues/Modals.htm
http://www.englishpage.com/modals/modalintro.html
http://www.tlumaczenia-angielski.info/angielski/modals.htm

http://m.youtube.com/watch?v=yGO_bRR66SM
http://m.youtube.com/watch?v=ozfFW4f4H3g