"create a complete declarative sentence that incorporates a personal possessive pronoun, a demonstrative pronoun, and describes an event."
can someone give me an example?

Well, first lets start with what a declarative sentence is. the kind of sentence that makes a statement or “declares” something. For example “He walks dogs." 

A possessive pronoun indicates possession, for example mine, yours, hers, theirs.

A demonstrative pronoun is a pronoun that identifies and specifies a noun or pronoun. This, that, these, those.

Knowing this, you can make a sentence using them all.

"He ate his yogurt that he had been waiting to eat for days."

The answer is holding on to the past