What is the correct word in this sentence and please explain: Wasn't it (he,him)?
Wasn't it he?
The nominative case in used whenever one employs the linking verb to be, whether he is the subject or the predicate nominative. However, since this sentence is a question, it should be turned into a declarative sentence for purposes of analysis.
Thus, the sentence as a declarative one reads, It was not (wasn't) he.
It is the subject and the past tense of to be, was, is the predicate. The noun or pronoun that follows the linking verb was is called the predicate nominative, and by definition, it must be in the nominative case. He is the nominative case of the third person singular masculine pronouns.
Because linking verbs are like = signs, the nominative case is required on either side since both parts are the equivalent of each other. Thus, sentences can be reversed and still make sense. It was not he = He was not it.
To answer this question appropriately, you must identify the subject of the sentence.
Since this sentence is in the form of a question, change it into a statement: 'It was not him.' In this sentence it is easy to identify "IT" as the subject and "HIM" as an object.
When deciding whether to use he/him in a sentence, you must know how the word is being used (object/subject). In this case, the the word in question is the object and you should use HIM.