What part of speech must a word be if its plural?

Answer 1

In English, only nouns and verbs can be plural in form. Adjectives, adverbs, pronouns, conjunctions, and prepositions do not change their form.

For example:

'The house is built of stone'. (Singular noun and verb.)

'The houses are built of stone.' (Plural noun and verb.)

Some adjectives and pronouns are plural in meaning, for example 'many' and 'we'. However, I take it that you are enquiring about words that have both a singular and a plural form.