What is a morpheme?
The smallest unit of language that carries meaning — smaller than a word and distinct from a syllable, which is a unit of sound rather than of meaning.
The basic division:
Free morphemes can stand alone as words: cat, run, happy.
Bound morphemes cannot: -s, un-, -ness, -ing. They must attach to something.
Why it is not the same as a syllable. Cats is one syllable and two morphemes — cat plus the plural -s. Water is two syllables and one morpheme, because wa- and -ter mean nothing separately. Counting syllables and counting morphemes give different answers, and the distinction matters.
The two kinds of bound morpheme:
Inflectional morphemes change a word's grammatical form without changing what it fundamentally is or its part of speech: plural -s, past tense -ed, comparative -er. English has very few — around eight — which is unusual, and is why English relies so heavily on word order.
Derivational morphemes create new words, frequently changing part of speech: happy → happiness (adjective to noun), nation → national → nationalise → nationalisation. This is how English generates vocabulary.
Why the concept is useful:
It explains how vocabulary is learned. Recognising -ology, bio-, anti-, -able lets a reader decode unfamiliar words rather than memorising each — which is why morphology instruction has a real effect on vocabulary growth.
It explains languages that behave very differently. Analytic languages such as Mandarin use few bound morphemes and rely on separate words. Synthetic languages pack many morphemes into one word, and polysynthetic languages can express with a single word what English needs a sentence for. This is why word counts across languages are not comparable.
Allomorphs are variant forms of the same morpheme: the English plural is pronounced differently in cats, dogs and horses — three sounds, one morpheme.
Some morphemes are not segments at all, such as the vowel change in goose/geese.