My friends phones are all new (more than one friend) / (where does the apostrophe go?)
My FRIENDS' phones (the phones belong to my friends, more than one friend) / no apostrophe - 'phones' is a plural noun
Its wagging its tail (where does the apostrophe go?)
IT'S (it is) wagging / no apostrophe - second 'its' is a possessive pronoun
Those are childrens clothes (where does the apostrophe go?)
CHILDREN'S clothes (the clothes belong to the children) / no apostrophe - 'clothes' is a plural noun
I am so happy to see you (can you combine any words with an apostrophe?)
I'M = I am
Thats Katys book (where does the apostrophe go?)
THAT'S = that is / KATY'S (the book belongs to Katy)
Mrs. Hank marked _________ homework. (the student)
Mrs. Hank marked the student's homework.
My brothers teachers classes (the classes taught by ONE teacher of ALL of my brothers) / (where does the apostrophe go?)
My BROTHERS' TEACHER'S classes
Its sad that the cat broke its leg (where does the apostrophe go?)
IT'S (it is) sad / no apostrophe - second 'its' is a possessive pronoun
The car belongs to Mr. Woo
Mr. Woo's car
My uncles wife is sick (where does the apostrophe go?)
UNCLE'S wife (wife of my uncle)
My friends bag is new (one friend) / (where does the apostrophe go?)
FRIEND'S bag (the bag belongs to my friend, one friend)
Shes 16 years old (where does the apostrophe go?)
SHE'S = she is / no apostrophe - 'years' is a plural noun
My brothers teachers classes (the classes taught by ONE teacher of ONE of my brothers) / (where does the apostrophe go?)
My BROTHER'S TEACHER'S classes
Thats my parents car (both parents) / (where does the apostrophe go?)
THAT'S = that is / my PARENTS' car (the car belongs to my parents)