In a grammar guide, I’ve read that “nominative case signals that the pronoun involved is functioning as the subject of a verb or as a subject complement.” Then it goes on to give an example how it is used as a subject complement:
The only contestants still tossing gnomoes were Berenice and he.
However, I still don’t have a clue waht a subject complement is. May I ask what exactly is a subject complement?
Hi Cooliegirly
A subject complement refers back to, modifies or completes the subject of a sentence. And a subject complement comes after a “linking verb”. A sentence where the main verb is be always provides a good example for subject complements. 
When the subject complement is a pronoun, it must be nominative (as your grammar book mentioned ;)). Subject complements can also be nouns, adjectives, or groups of words behaving as a noun or adjective.
Amy
Finally, I get it! Thank you, Amy.