Make most singular nouns possessive
by adding 's:

John's car
the boy's room
my friend's father
my neighbor's car

If a singular noun ends in s or ss, it's
possible to add 's, but many people
feel that adding only an apostrophe
looks better and is clearer--especially
if the noun ends in ss:

Dennis's car / Dennis' car
Charles's children / Charles' children
Russ's room / Russ' room
Bess's father / Bess' father

If a noun is plural and ends in s, make it
possessive by adding only an apostrophe:

the boys' room
my parents' jobs
the students' opinions
his brothers' names

For common words with irregular
plurals, add 's:

the children's toys
women's gymnastics
men's clothing

Possessives for words ending with s are
pronounced the same if they use 's or ':

Dennis's / Dennis' = 'Dennisuz'

Charles's / Charles' = 'Charlesuz'

Russ's / Russ' = 'Russuz'

Bess's / Bess' = 'Bessuz.'

Singular possessive nouns ending with 's
and plural possessive nouns ending with s'
also have the same pronunciation:

your son's / your sons' =
the same pronunciation

the neighbor's / the neighbors' =
the same pronunciation

my brother's / my brothers' =
the same pronunciation

For singular / plural nouns, adding 's or '
can change meanings:

his son's room = the room belonging to
his son

his sons' room = the room belonging to
his sons

the neighbor's children = the children
of only one neighbor

the neighbors' children = the children
of two or more neighbors

The combination 's doesn't always show
possession: 's can also mean is or has.
To know which meaning is intended,
look at the words following 's:

Mary's father ( 's shows possession )

Mary's intelligent ( 's = is )

Mary's working hard. ( 's = is )

Mary's been there many times. ( 's = has )

Mary's had problems. ( 's = has )