Forming the English Possessive

Possession need not refer to literal ownership, but it's a good place to begin. 

Dave owns some guns, so we can speak of Dave's guns. 

Mary has a pick-up, so we can speak of Mary's pick-up. 

But we can also speak of John's children and Sue's grandmother, even though John does not own his children, nor Sue her grandmother.

A better way to think of possession in English is as a relationship that can also be expressed syntactically, through a phrase with the word of.

Dave's guns

=

the guns of Dave

Mary's pick-up = the pickup of Mary
John's children = the children of John
Sue's grandmother = the grandmother of Sue

Formation of the Possessive 

In writing, there are three rules to apply:

1.

You can form a possessive whenever you can also form an of expression that has the same meaning:

Fred's kitchen is very clean

=

The kitchen of Fred is very clean.

Judy's house is over there = The house of Judy is over there.

Incorrect:

The horse's are eating now = The __?__ of horse are eating now.

Whatever goes on the end of the of phrase is the possessor, which may be any noun.

2.

Form the possession by adding 's to the end of the possessor, even if that word is plural.

3.

If this results in the sequence s's, drop the last s.

Apply all these rules to all forms of the possessive, and you won't make a mistake.

(For singular forms of the possessive, this last rule is not required, but it's OK to apply it.)