Some books/sites/language tutors assert that an apostrophe is used for contraction of is,are,am,was,were,has,have etc, such as, he is = he's she is = she's they are = they're others imply that they are only used to show possession, such as the boy's bike my mother's stove and some even imply that they are used as both. i'm confused, which one is true? or is this another US/British english? please advise. thank you.