The apostrophe has 2 functions:
1. Contractions
2. Possession
Contractions
The apostrophe is used to indicate the missing letter, eg:
Beware :
Possession
When the possessor is single (ie, just one person or thing), we indicate possession by using an apostrophe followed by the letter s:
When the possessors are plural (ie, more than one person or thing), we indicate possession by placing an apostrophe after the final s:
However:
when a word changes completely in the plural, the apostrophe remains before the s:
Apostrophes are not required where there is no possession eg:
If the subject is followed by a verb an apostrophe will probably not be needed, if it is followed by a noun then one may be required
See also:
its = belonging to it (as in his, hers and its)
it's = it is or it has (its' does not exist)
your = belonging to you
you're = you are
there = over there
their = belonging to them
they're = they are
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