The Verb To Be has the same Case after it as before it.
In such instances, the noun or pronoun after the verb IS not in apposition with the noun or pronoun before it, but is a part of the predicate of the verb. Thus….
James is commander of the troops.
Here, COMMANDER should be parsed as nominative after the verb is. The same meaning would be conveyed by the expression (James commands the troops.) so that the word commander is really a part of the predicate.
This rule applies to all the variations of the verb TO BE such as am, art, is, was, were, etc. It applies also to the verb become and to several other intransitive verbs and also to the passive voice of some transitive verbs such as to be named to be called.
The verb to be in the infinitive moot! used as a noun may have a noun or a pronoun after it without any other noun before it as….To be a good man is not so easy a thing as many people imagine.
Here MAN should be parsed as used indefinitely after the verb TO BE without saying what its case is. The infinitive mood of many other intransitive verbs and likewise the infinitive passive of some transitive verbs may also have a noun or a pronoun after
them used indefinitely as…..