NOUNS WITH ONE FORM:

Some nouns ending in s have the same singular and plural form.

  • This means of transport saves energy./Both means of transport save energy.

  • This species of insect is quite rare. /All these species of insect are quite rare.

NOUNS WITH ONE FORM:

crossroads, means, series (e.g. a series of TV documentaries), species (kind, type) Works (a factory) and headquarters (a main office)

take either a singular or a plural verb.

The steel works has/have closed down.