I need help in writing a sentence using the possessive form of the nouns.Peter owns a gray unbrella. That story is from Cindy. The eyes of Peter...

I think you must be having problems identifying which noun
in each sentence needs to be made possessive. This can be determined by thinking about
the meaning of the sentence.

In your first sentence, an
umbrella cannot own anything; it must be that the umbrella belongs to Peter. To make the
noun (and proper name) Peter possessive, add
's--Peter's umbrella is
gray.

Cindy does not come from a story--the
story comes from Cindy, so the sentence could read: That is Cindy's
story.

In the third sentence, we hope that
the eyes are green instead of Peter being green!  Since the eyes belong to Peter, your
sentence would read: Peter's eyes are
green.

In your fourth sentence, you would
add 's to Sandy (the owner of the car) and the sentence
would be: This is Sandy's
car.

You would not change the "y" in Sandy
to an "i" and add "es" because you are using the apostrophe to indicate possession of
the car by Sandy; you are not indicating that there are more than one Sandy--"Sandies"
would be a group of something all called
Sandy.