Which type of affix serves a grammatical function without changing the part of speech?

Inflectional affixes are specifically designed to serve a grammatical purpose while maintaining the part of speech of the base word. These affixes modify a word to express different grammatical features such as tense, mood, voice, aspect, person, number, gender, and case. For example, adding "-s" to a noun to make it plural ("cat" becomes "cats") or adding "-ed" to a verb to indicate the past tense ("walk" becomes "walked") are both functions of inflectional affixes.

This is in contrast to derivation affixes, which can change the part of speech of a word. For instance, adding "-ly" to the adjective "quick" forms the adverb "quickly." Descriptive and combinational affixes are not recognized standard terms in linguistics for types of affixes, which reinforces that the main category distinguishing inflectional affixes is their grammatical role without affecting the word's part of speech.