It is unacceptable to submit an essay from a previous course.
I am having a hard time finding the verb and the subject of this sentence. I searched online. From what I understood the answer should be "essay" "submit" but not sure.
"Is" is the main verb of the sentence. "It" is the subject.
This is an example of the common "dummy pronoun" structure in English. "It" has no meaning, and is just used as a kind of placeholder in the sentence.
It is good to eat breakfast every morning.
It is smart to check your sources before submitting an answer
It is a bad idea to stick your hand in the lion's cage.
It is unacceptable to forget my birthday.
and so on.