's

SINGULAR NOUN + 's

my friend's house

SINGULAR NOUN ENDING IN -S + ' / 's

Socrates' ideas / Socrates's ideas

PLURAL NOUN ENDING IN -S + '

my parents' car

IRREGULAR PLURAL NOUN + 's

the children's room

's is usually used with:

PERSONAL NAMES: Mary's house

PERSONAL NOUNS: the boy's new shirt

COLLECTIVE NOUNS: the government's decision

HIGHER ANIMALS: the horse's tail

GEOGRAPHICAL AND INSTITUTIONAL NAMES:
Europe's future

TEMPORAL NOUNS: today's newspaper

NOUNS OF SPECIAL INTEREST TO HUMAN ACTIVITY:
the brain's solid weight