A
We use -‘s (apostrophe + s) mostly for people or animals:
 – Tom’s computer isn’t working. (not the computer of Tom)
 – How old areChris’s children? (not the children of Chris)
 – What’s Tom’s sister’s name?
 – Be careful. Don’t step on the cat’s tail.
Note that you can use -‘s without a noun after it:
 – This isn’t my book. It’s my sister’s. (= my sister’s book)
We do not always use -‘s for people. For example, we would use of … in this sentence:
 – What was the name of the man who called you? (“the man who called you” is too long to be followed by -‘s)
Note that we say a woman’s hat (= a hat for a woman), a boy’s name (= a name for a boy), a bird’s egg (= an egg laid by a bird), etc.
B
With a singular noun we use -‘s:
 – my sister’s room (= her room – one sister)    |    Mr. Carter’s house (= his house)
With a plural noun (sisters, friends, etc.) we put an apostrophe at the end of the word (-s’):
 – my sisters’ room (= their room – two or more sisters)
 – the Carters’ house (= their house – Mr. and Mrs. Carter)
If a plural noun does not end in -s (for example men/women/children/people) we use -‘s:
 – the men’s changing room    |    Mr. and Mrs. Carter’s house
C
For things, ideas, etc., we normally use of (… of the book / … of the restaurant, etc.):
 – the government’s decision    or    the decision of the government
 – the company’s success        or    success of the company
It is also possible to use -‘s for places. So you can say:
 – the city’s streets    |    the world’s population    |    Brazil’s largest city
E
You can also use -‘s with time expressions (yesterday / next week, etc.):
 – Do you still have yesterday’s newspaper?
 – Next week’s meeting has been canceled.
In the same way, you can say today’s / tomorrow’s / tonight’s / Monday’s, etc.
We also use -‘s (or -s’ with plural words) with periods or time:
 – I’ve got a week’s vacation starting on Monday.
 – Sally needs eight hours’ sleep a night.
 – Brenda got to work 15 minutes late but lost at hour’s pay.
  
  
  
  

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