
Possessive: Shows ownership.
The student's book (one student)
The students' books (multiple students)
It's vs Its:
It's = it is / it has. ALWAYS a contraction.
Its = possessive (like his, her). NO apostrophe.
Who's vs Whose:
Who's = who is / who has
Whose = possessive
They're / Their / There:
They're = they are
Their = possessive
There = location / existence
SAT rule: Apostrophes NEVER make plurals. "The Smith's" is WRONG for a plural. "The Smiths" is correct.
Reference:
TaskLoco™ — The Sticky Note GOAT