English is among the simpler languages in the way it forms plurals of simple nouns: in most cases it just sticks an “s” to the end of the noun. But some kinds of noun are not simple, and forming their plurals is not always straight-forward. Commonest among these are the compound nouns, which are comprised of two or more words: e.g., two nouns, or a noun and a qualifying word (an adjective, for instance), or a noun and a phrase. In such cases the compound noun is easily seen as a substantive noun, plus the qualifying word or words. To form the plural in such cases, an “s” is added to the substantive noun, not to the qualifiers. Thus, we have:
SINGULAR — PLURAL
- Chief of staff — Chiefs of staff
- Commander-in-chief — Commanders-in-chief
- Man-of-war (a ship) — Men-of-war
- Head of State — Heads of State
- Mother-in-law — Mothers-in-law
- Passer-by — Passers-by
- Field Marshall — Field Marshalls
In this context of pluralization, note that “staff” is one of those words that refer to a group (not a single person or item): like “team,” “cast” (of a play/movie), “faculty,” etc.
Such nouns may be viewed as singular or plural, depending on the context.
- In “His staff is assembled,” the group is viewed as acting together, as one; but
- In “His staff are engaged in different aspects of the project,” the staff are treated as different individuals.