This question and answer comes from The Grammar Logs #464 at Capital Community College:
Please help me with the following sentence:
Mingling with the crowd (was, were) several Secret Agents.
Is “mingling with the crowd” a gerund phrase acting as subject? If so, is it singular, therefore, requiring the singular verb “was”? or is “several Secret Agents” the subject requiring the plural verb “were” or is it a predicate nominative? If the subject is indeed the latter, what is the phrase “mingling with the crowd”? What is the sentence pattern here?
I don’t know if there is really such a thing as a simple inverted sentence — a sentence in which the verb precedes the subject — but that’s what you’ve got here. The subject, “agents,” is plural and demands a plural verb, “were.” You can regard “Mingling with the crowd” as a predicate nominative or as the present participle accompanied by a prepositional phrase.
So there. I’m sure you all really wanted to know.