The completed message is: "Listen up! If I find out who took my favorite blue sweater, they are going to hear from me! That sweater isn't yours, it's mine!"

Let's break down the "I" family:

• I is the subject (the person doing the action: I find out).

• my is a possessive determiner (it sits right next to a noun to show ownership: my sweater).

• me is the object (it receives the action or follows a preposition: from me).

• mine is a possessive pronoun (it stands alone and completely replaces the noun: it's mine = it's my sweater).