To form the possessive...
The car of John
Functions of the possessive
It shows a relationship of belonging between one thing and another.
Fixed expression
Men's clothes
John's car
It is John's car.
'Belonging to' or 'ownership' is the most common relationship the possessive expresses.
add apostrophe + s to the noun. If the noun is plural, or already ends in s, just add an apostrophe after the s.
Possessive
a day's work