You should use an apostrophe to form the possessive 
  case of a noun or to show that you have left 
  out letters in a contraction. Note that you 
  should not generally use contractions in formal, academic writing. 
- The convertible's engine has finally died.  (The noun
"convertible's" is in the possessive case)
 
- I haven't seen my roommate for two weeks.  (The verb
"haven't" is a contraction of "have
not")
 
 
To form the possessive of a
plural noun ending in "s," simply place
an apostrophe after the "s." 
- He has his three sons' futures in mind.
 
- In many suburbs, the houses' designs are too much alike.
 
 
Possessive pronouns -- for example, "hers,"
"yours," and "theirs" -- do not take
apostrophes.  This is the case for the possessive pronoun "its" as well: when you write "it's" with an
apostrophe, you are writing a contraction
for "it is." 
- The spaceship landed hard, damaging its radar receiver.
("its" is the possessive pronoun)
 
- It's your mother on the phone. ("it's" is the contraction
of "it is")
 
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