What does an inflectional suffix do?

An inflectional suffix serves the purpose of modifying a word while retaining its original part of speech. This means it allows for grammatical variations within the same category without altering the fundamental meaning or function of the word. For example, when adding the suffix '-s' to the noun 'cat', it becomes 'cats', indicating a plural form, but it remains a noun.

While some suffixes indicate specific grammatical features—such as transforming verbs into their past tense or deriving plural forms—the key characteristic of inflectional suffixes is that they help convey different grammatical aspects (like tense or number) while still preserving the base word's syntactic category. Therefore, the choice indicating that inflectional suffixes form new words which maintain the same part of speech accurately captures this function.

The options which focus solely on indicating past tense or plural form are too restrictive since inflectional suffixes encompass a broader range of grammatical modifications, not limited to those functions alone. The first choice's implication of altering meaning to the opposite does not relate to the function of inflectional suffixes, as these do not inherently change the meaning of a word to its antonym.