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.
a noun
noun, plural.
A plural noun.
A plural noun
plural noun
plural noun
noun, plural
It is a plural noun.
Plural noun.
A plural noun
The word power is a noun. The plural form is powers.
The word railroads is a plural noun.