A predicate nominative is a noun or pronoun that follows a linking verb and refers to the subject of the verb.
Examples
In these examples, the underlined words are predicate nominatives:
They are the ones who play sparingly in the preseason … [TampaBay.com]
They are the men who fans pay to watch and that the TV cameras and journalists love to focus on. [Aljazeera]
She is a former member of the U.S. Soccer Federation Under-15 Women’s National Team … [ESPN]
He believes he is who his reputation says he is … [Astrochicks]
To the ark
The sentence in question is: ” Drugs aren’t one of the problems. Shouldn’t the predicative nominatve agree with the subject? The sentence before that is, “We have 99 problems.