RULES OF THUMB

Let's play with the letter "S" 

1 - The commonest way of forming the plural of nouns is to add "S" to the singular:
e.g. Book - Books - House - Houses - Boy - Boys

2 - The Third Person Singular of the Present Tense ends in "S":
e.g. She looks gorgeous in that red dress
He speaks English and French
She drives her car recklessly

3 - The apostrophe before a final "S" stands for a Possessive Case:
e.g. This is Mary's book
That is Mr Brando's automobile

4 - The apostrophe before a final "S" also stands for IS:
e.g. Peter's a nice guy. Peter is a nice guy
He's waiting for you at the airport. He is waiting for you at the airport

5 - The apostrophe before a final "S" stands for HAS:
e.g. He's (he has) gone to Paris on his Annual Leave
She's (she has) been sleeping for 24 hours
 
6 - The apostrophe before a final "S" stands for LET'S = LET US:
e.g. She is tired, let's take her home.
Forget the football game, let's go to the movies instead.

7 - The apostrophe after a final "S" indicates the possessive form of the plural:
e.g. The Girls' results were better than the Boys'.