Possessive 'S' by Viv Quarry (www.vivquarry.com)
We mainly use possessive 's' with:
I stayed at my uncle's brother's house.
*1 I went to my parents' house last weekend.
*2 You can sleep in the children's room.
*3 The boss' decision / the boss's decision. (in both cases, the pronunciation is /BO SIZ/ )
Don't eat those! They're the dog's biscuits! (= 1 dog)
*1 Don't eat those! They're the dogs' biscuits! (= 2 or more dogs)
*2 The sheep's wool was very thick.
*4 The tiger opened its mouth.
3. In some time expressions
I 'm taking a month's holiday.
*1 They're giving me three months' salary.
*1 With plural nouns, the apostrophe comes after the 'S' (see note 2 below).
*2 Unless the noun is an irregular plural noun.
*3 When a word ends in 's' you can either add 's or only an apostrophe.
*4 Possessive 's' with 'it' has no apostrophe (it's = 'it is' or 'it has')
We also use possessive 's with:
groups of people: the government's decision.
countries and continents: Britain's new parliament / Europe's economic problems
planets: the earth's core / the world's natural resources
companies: BP's profits
Apart from possessive 's', English also uses compound nouns and prepositional phrases:
We went to Copacabana Beach.
We kept the wine bottle. (= the empty bottle)
We bought a bottle of wine. (= it was full of wine)