Apostrophes make nouns possessive.
For singular nouns, always add apostrophe + -s
The boy’s ball = The ball belonging to the boy
The dress’s buttons = The buttons on the dress
For plural nouns, always add -s + apostrophe, or -es + apostrophe if the singular version ends in -s (e.g. dress).
BUT if the plural form of a noun does not end in -s (e.g. feet, children, geese), then add apostrophe + -s only.
The boys’ ball = The ball belonging to the boys
The dresses’ buttons = The buttons on the dresses
The children’s game = The game the children are playing.
Pronouns: no apostrophe = possessive
It’s vs. Its
It’s = It is
Its = Possessive form of it
Its’/Its’s = Do not exist
Note: when the ACT tests it’s vs. its, the answer is virtually always its because that is the version students tend to have the most difficulty using correctly.
Incorrect: London is a city known for it’s (it is) many tourist attractions.
Correct: London is a city known for its many tourist attractions.
Incorrect: London is a popular tourist attraction; in fact, its among the most visited cities in the world.
Correct: London is a popular tourist attraction; in fact, it’s among the most visited cities in the world.
They’re vs. Their vs. There
They’re = they are
Their = possessive form of they; plural of its
There = a place
Correct: London and Paris are two of the most famous cities and Europe, and they’re (they are) known for having many tourist attractions.
Correct: London and Paris, two of the most famous cities in Europe, are known for their many tourist attractions.
Correct: In the nineteenth century, Paris was considered the capital of the art world because so many famous painters lived and worked there.
Who’s vs. Whose
Who’s = Who is
Correct: Barbara McClintock is a scientist who’s (who is) best known for her discovery of “jumping” genes.
Whose = Possessive of who
Correct: Barbara McClintock is a scientist whose discovery of “jumping” genes helped earn her the 1983 Nobel Prize in Physiology or Medicine.
Note that whose, unlike who, can be used for both people and things/places.
Correct: London is a city whose many museums, palaces, and monuments make it a popular tourist destination.