Not "my sister room" — say "my sister's room". The 's is what shows the room belongs to her.
Try it: This is my ___ room. (it belongs to my sister) (sister's / sister / sisters) Answer: sister's — To show the room belongs to her, you need the 's. You left it off, add it: my sister's room.
Plural ending in -s: just add '
When the plural already ends in -s, add only an apostrophe, no extra s.
my parents' house
the students' books
the teachers' room
Not "the students's books" — say "the students' books". There is already an -s, one apostrophe after it is enough.
Try it: These are the ___ books. (they belong to the students) (students' / students's / student's) Answer: students' — 'Students' already ends in -s, so you add only an apostrophe, not another s: the students' books.
Irregular plurals: add 's
Some plurals (children, men, women) don't end in -s, so they take 's like one owner.
children's toys
men's clothes
women's shoes
Not "the childrens' toys" — say "the children's toys". Children has no -s, so it takes 's, not an apostrophe at the end.
Try it: We packed the ___ toys. (they belong to the children) (children's / childrens' / childrens) Answer: children's — 'Children' does not end in -s, so it takes 's like one owner: children's toys (not 'childrens').
Where does the apostrophe go?
Find the owner first, then put the apostrophe after the owner. One sister is sister's, and two sisters are sisters'.
one sister: my sister's room
two sisters: my sisters' room
one friend: my friend's phone
Not "my sister's room (two sisters share it)" — say "my sisters' room (two sisters share it)". In writing, the apostrophe after sisters is what shows the room belongs to two people.
Try it: Mei has two brothers. This is her ___ room. (both brothers sleep there) (brothers' / brother's / brothers) Answer: brothers' — Two brothers own the room, so the apostrophe goes after brothers: her brothers' room. 'Brother's' would mean only one brother.
's = is, or 's = owner?
's does two jobs. After a name, 's before a describing word is short for is. Before a thing, 's shows the owner.
Liam's tall. = Liam is tall.
Liam's bag = the bag belongs to Liam
She's happy. = She is happy.
Not "Sofia's tired means the tired one belongs to Sofia" — say "Sofia's tired means Sofia is tired". Look at the next word. A describing word means is, and a thing means the owner.
Try it: Hiroshi's phone is on the table. What does the 's do here? (it shows the owner / it is short for is / it makes it more than one) Answer: it shows the owner — The next word is a thing, phone, so the 's shows who owns it. If the next word were a describing word, as in Hiroshi's late, the 's would be short for is.
Try it: Remember the 's. This is ___ bag. (it belongs to Liam) (Liam's / Liam / Liams) Answer: Liam's — One owner, Liam, so add 's: Liam's bag. (One owner always takes 's.)