Amy came back from work, and when she opened the door, the phone immediately started to ring.
Immediately is an adverb.
If the sentence is: I will go to the store immediately...then "immediately" is an adverb. It answers the "when" requirement. "Immediately" is never a preposition.
That sentence is a declarative sentence, a sentence that makes a statement.
The phrase "Unpack immediately." is an imperative sentence. The understood subject is "you," so the sentence means "(You) unpack immediately." It is a command.
"Immediately" is the adverb.
Yes! Immediately exit the building. Immediately turn on your television. Immediately look up Mitch Longley. And so on and so forth.
We need to evacuate this building immediately!
Imperative A+
I want you to run to the store immediately and get some ice cream.
I wish she had been immediately enamored of me.
I must speak with a haberdasher immediately.
Emphatic