When do we add apostrophy by word end?

Usually:

a) To mark possession: e.g. the man's book, the bride's dress, the car's engine; the players' changing-room. Note the relative position of the apostrophe on the plural: one man, bride or car but many players.

b) In certain abbreviations: it's (= it is), that's (=that is), and so on. NB the difference between its and it's. "Its" is the possessive for "it", as this rather contrived example shows: "The car's engine failed. It's a write-off because itsblock has cracked."