The use of a complement with a linking verb causes problems, especially with pronouns. The general rule for using the linking verb requires us not to change the pronoun, wherever its position in the sentence.
Oh yeah? Look at these three examples:
1. He is the problem, or, The problem is he.
2. They are the workers, or, The workers are they.
3. It is I, or, I am it.
2. They are the workers, or, The workers are they.
3. It is I, or, I am it.
In example 1, He is the problem sounds natural to the native ear, but The problem is he does not. Most articulate people would say The problem is him.
In example 2, the same problem arises. They are the workers sounds fine, but The workers are they does not, and I doubt I ever heard the sentence inverted to The workers are them. We just start that sentence with the pronoun they.
I saved the best for last. In example 3, both sentences represent a fluency problem. We all say It is me, not It is I. While I am it may sound strange, notice it would sound far worse if I said I is it, which I am saying in the "correct" It is I. And can you imagine saying It am I, which the rule seems to call for?
Don't obsess over this arcane rule. When writing, if it sounds right to you, write it that way.
**
Read previous posts in this series:
Part 1: Grouping and Dropping Prepositional Phrases
Part 2: Dropping Pronouns for Clarity
Part 3: Dropping Pronouns for Conciseness
Part 4: Avoiding—No, Managing—the Comma Splice
Part 5: Remedying Run-ons
Part 6: Freeing Fragments
Part 7: Placing Adverbs
Part 8: Placing Adjectives
Part 9: Placing Prepositional Phrases
Part 10: Placing Conjunctions
Part 11: Placing Interjections
Part 12: Placing Verbs
Part 13: Placing Nouns