I want you to run to the store immediately and get some ice cream.
The medicine had an immediate effect
The phrase you are thinking of is effective immediately, not effectively immediately. This means that something (usually a rule or procedure) is in effect right now, with no waiting period.
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.
I shall immediately fire off a severely chastening letter to the supervisory staff of this hotel!
Emphatic
The pronoun in the sentence is he, the subjective case functioning as the subject of the sentence.
Mary's remark was so insubordinate, she was fired immediately.
The dog immediately started scenting a bone.
The teacher saw the funnel cloud on the horizon and called to the children playing in the yard: "Come in, immediately!"
If the fire alarm goes off, exit the building immediately!
I drank the healing elixir and immediately felt better.
If its a statement like, Leave us immediately. that's how you use like an umm.. emotion or like your yelling at someone
When the herring jumped out of the water, it was immediately eaten by a seagull.
When they realised that he was innocent his lawyers insisted that he should be freed immediately.
Example: Is that awful stench coming from your shoes? Go wash them immediately!
Oddly enough, I immediately saw the solution to the problem at hand.
Meet me at the rendezvous point, immediately. We were supposed to rendezvous at nine.
The phrase you are thinking of is effective immediately, not effectively immediately. This means that something (usually a rule or procedure) is in effect right now, with no waiting period.