Possessive determiners are the words we use to show who or what "owns" something. They sit right before a noun and, unlike possessive pronouns (like mine or yours), they cannot stand alone.
Think of them as the "security guards" for nouns—they identify exactly which noun we're talking about based on its relationship to someone.
The Core List
Here is the lineup of possessive determiners matched with their corresponding subject pronouns:

Subject Pronoun	Possessive Determiner	Example Sentence
I	My	That is my coffee.
You	Your	Is this your umbrella?
He	His	His car is in the shop.
She	Her	I borrowed her book.
It	Its	The dog wagged its tail.
We	Our	Our flight was delayed.
They	Their	Their house is on the corner.

Key Grammar Rules
Placement: They always come before the noun (or the adjective modifying the noun).
Correct: My blue shoes.
Incorrect: The shoes my.
No Apostrophes: Unlike possessive nouns (e.g., Sarah's), possessive determiners never use apostrophes. This is the #1 source of grammar errors.
Gender and Number: The determiner matches the owner, not the object being owned.
If a woman owns ten dogs, we say "her dogs." (Singular owner = singular determiner).
Common Pitfalls (The "Avoid These" List)
The biggest hurdles with possessive determiners are homophones—words that sound the same but are spelled differently.
Its vs. It's
Its: Possessive (The tree lost its leaves).
It's: Contraction of "it is" or "it has" (It's raining).
Their vs. There vs. They're
Their: Possessive (Their cat is mean).
There: Location (Go over there).
They're: Contraction of "they are" (They're coming over).
Your vs. You're
Your: Possessive (Wash your hands).
You're: Contraction of "you are" (You're doing great).
A Quick Trick
If you aren't sure if you should use a possessive determiner or a contraction, try replacing the word with "his." If the sentence still makes sense (even if the meaning changes), you need a possessive determiner.
Example: "Is that your/you're phone?"
Test: "Is that his phone?" (Works! Use your).
Example: "Your/You're going to be late."
Test: "His going to be late." (Nonsense. Use you're).