Is a singular subject with a plural predicate nominative ever ungrammatical (or unacceptable)?
I ask because several non-native speakers have told me that a singular subject with a plural predicate nominative in English is NEVER acceptable, or at least usually unacceptable, and, as a native speaker, I am baffled by this stance.
Just doing a few grammar thought experiments, I get sentences like those below, which seem perfectly fine.
This book is 300 pages.
One dollar is 100 cents.
America is 50 states.
Is the issue that the subject and predicate are different nouns? What if the nouns are the same, like below?
This book is five books.
Still seems acceptable. Maybe the one book is a compendium of multiple individual books.
So, when is a singular subject with a plural predicate nominative unacceptable? Always? Never? If sometimes, in what grammatical situations or contexts?