main office vs plural

main office and plural both are nouns.

Word	Noun	Adjective	Verb	Adverb
main office	Yes	No	No	No
plural	Yes	Yes	No	No

main office (noun)	plural (noun)
(usually plural) the office that serves as the administrative center of an enterprise	the form of a word that is used to denote more than one

main office (adjective)	plural (adjective)
	grammatical number category referring to two or more items or units
	composed of more than one member, set, or kind