Anon: Back is an adverb. Adverbs have no plural form. The subject of the sentence is "you" - it is a command. The direct object is "it" The verb only has an "s" ending when the subject is 3rd person singular: he, she, or it. Example:
He backs the car carefully into the garage. (Here "back" is a verb, and "he" is the subject, so you see the "s" on the end of "back
No. It's an imperative sentence. [You] give it back. Back is not a verb. The verb is "give." Please give it to me. Joe gives it to me. "It" is the object, not the subject. It does not determine the "person and number" of the verb. The verb is "to give." You give, he gives, it gives. The subject of the sentence is You -