Morphemes divide into two major functional types: inflectional and derivational. Inflectional morphemes mark grammatical categories like tense, number, case, or agreement without changing a word's basic category or core meaning (e.g., 'walk' → 'walks', 'walked'). Derivational morphemes create new words, often changing grammatical category or substantially altering meaning (e.g., 'teach' → 'teacher', 'happy' → 'unhappy'). Affixation (prefixes, suffixes, infixes, circumfixes) is the most common morphological operation cross-linguistically.
Create two-column tables comparing inflectional vs derivational affixes in English. Then examine a language with rich inflection (Latin, Russian) to see how far inflectional morphology can extend.
In morphological structure, you learned that words can be decomposed into morphemes — the smallest meaning-bearing units of language. Now the question is: not all morphemes do the same thing, so how do we classify them? The most fundamental distinction is between inflectional and derivational morphemes, and the difference comes down to what each type of morpheme *does to the word it attaches to*.
Inflectional morphemes adjust a word to satisfy the grammatical requirements of its context. When you say "she walks" instead of "she walk," the -s is not creating a new word; it is tuning the existing verb "walk" to agree with a third-person singular subject. The word is still "walk" — same category (verb), same core meaning. English has relatively few inflectional morphemes: plural -s, possessive 's, third-person -s, past tense -ed, progressive -ing, and a handful of others. Many other languages have far richer inflectional systems. Latin verbs carry suffixes marking person, number, tense, mood, and voice simultaneously. Russian nouns inflect for six grammatical cases. The point is that inflection is grammatically obligatory: the syntax demands it.
Derivational morphemes, by contrast, create new words. When -er attaches to "teach," the result "teacher" is a new noun — a distinct lexical item with its own entry in the mental lexicon, its own semantic properties, and its own ability to be inflected further ("teachers"). Similarly, the prefix un- turns "happy" into "unhappy" — a new adjective with a meaning not simply recoverable from "not" + "happy" (consider: "unhappy" is stronger than merely "not happy"). Derivation is creative and productive, but not obligatory: nothing in the grammar of English requires you to create "teacher" from "teach." You choose to.
One important nuance — and a common misconception — is that derivation does not always change grammatical category. The suffix -dom creates "kingdom" (noun) from "king" (noun), and "boredom" from "bore" (noun or verb, with nominal interpretation selected). What makes -dom derivational is that it creates a new word, not that it changes the category. The test is whether the output has to be registered separately in the mental lexicon, with potentially idiosyncratic meanings that cannot be fully predicted from the parts.
Finally, be cautious about generalizing from English. English has worn away most of its inflectional morphology over centuries; a verb like "cut" is identical in present, past, and past-participial forms. Languages like Turkish or Georgian use inflectional suffixes to encode information that English expresses through separate words or word order. Morphological typology — how languages vary in the amount and type of morphological work they do — is one of the richest areas of cross-linguistic comparison, and your understanding of the inflection/derivation distinction is the conceptual foundation you need to explore it.