Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

Number concord with subject complements

279c

In informal spoken language, speakers sometimes have a choice whether to use a singular or

plural verb when there is a difference in number between the subject and the complement:

singular subject

plural complement

Another topic of course 

is

words that have changed their meaning

. 

[talking of a car that is proving difficult to sell]

The only good thing 

are

the tyres

.

Normally, and in more formal styles, the subject determines the number of the verb .

Apposition

Adjective and noun phrases separated from the rest of the clause by punctuation

or intonation may function in a way very similar to a subject complement:

The crowd pressed closer, unwilling to miss a thing.

We’d just sit there, a couple trying to make it in the world.