I believe I have talked about the predicate nominative before, but if I recall correctly, it was just an excursive, cursory explanation of it. With that said, I still want to expound further on this precept so that there be no confusion per se.
As you are probably eminently aware, the title explains this concept of the predicate nominative—"Woe is I"—instead of the more traditional way of saying, "Woe is me". Yes, I know Shakespeare did say, "Woe is me" and I am in no way saying that he was wrong for having said it like this. In fact, standard English grammatical rules weren't established until the mid-seventeenth century so Shakespeare had been dead approximately half of a century before English was standardized; therefore he was not wrong when he said it in this way.
Furthermore, not to rend the grammatical rules along with the semantics of this construction, but using an objective pronoun following a copula is just informal; it in no way impugns the interlocutor in any way. In fact, it may enhance a dialogue by removing the fusty nature that a phrase like "It is I" might induce. With that said, though, in formal writing, it is important that the writer follow this rule because it does produce concinnity in the paper, semantically it makes more sense, and grammarians still say it is correct, but formal.
Remember that high school and college English papers are formal bodies of work; ergo they must be stylistically formal. One must also remember that an accusative pronoun needs an action verb for it to be used. A writer cannot use an accusative pronoun as the direct object of a copula because a copula is only linking the subject with the subject predicate. There also must be concinnity in the construction of a "to be" construction with its subordinate clause. For instance:
Example: It is you [who are going to rectify this situation.]
Above, the predicate nominative is "you"; therefore the verb following the interrogatory pronoun "who" in the subordinate clause must correspond with each other; therefore it would appear to be "you are" in this situation. Below is another example so that you can see how this works:
Example: It is she who owns that land.
Now, remember that this will only be easy to spot provided "to be" be conjugated or it have a subject. It can be very difficult to spot during such times when the subject should appear to be nonexistent. For example:
Example: I want [myself] to be him.
Here, "to be" has a subject and that subject is the direct object of the verb to want, but because it's a reflexive pronoun, in this situation, it is normally omitted. The infinitive here is linking the two objective pronouns so they must both be objective. Here are some more examples below:
Example: I pretend to be her.
Example: I pretend that I am she.
Example: It's better that it be they than it be I.
Example: Yesterday, I became he [the leader].
Example: The winner was thought to be I.
Example: He thought me to be him.
Everyone reading this should be aware that this is formal English—English that one would write more so than he would speak. More so, many of these constructions can, indeed, be avoided in a paper and I would strongly urge that a writer avoid these constructions because of their stilted, pedantic constructions, but they do have their place in grammar. It should be stated, though, that a construction such as the example below can bring a connotation of strength and may embolden its audience so this construction can definitely be wise to use in a formal, persuasive paper:
"It is we, the people, who have fought for centuries for freedom. It is we who have offered our souls, our blood, and our brethren to fight against persecution and injustice. It is we who have the power to decide what is right for us and it is we who will never give up until we have what is rightfully ours."
This is a term in linguistics called "anaphora". Winston Churchill was renowned for his use of anaphora in many of his speeches. Anaphora is very powerful in persuasive writing because it causes the reader to focus on its construction. The constant, recurrent use of "It is we" is enough to cause anyone's head to turn.
All right, well that is my in-depth explanation of the predicate nominative. I hope it might have helped you a little in understanding the antiphony of prescribed constructions versus proscribed constructions regarding this topic.