A morpheme is the smallest unit of language that carries meaning. The word unhappiness contains three morphemes: un- (negation), happy (core meaning), -ness (converts adjective to noun). Morphology, the study of morphemes and their combinations, is foundational to understanding word formation and a key construct in SLA research.
Free morphemes can stand alone as words: book, run, happy, the. They divide into content words (lexical morphemes: tree, walk) and function words (grammatical morphemes: the, and, if).
Bound morphemes must attach to another morpheme. These divide into:
Roger Brown (1973) discovered that L1 English children acquire grammatical morphemes in a predictable order. Dulay and Burt (1974) found a similar but distinct order for L2 learners, providing early evidence for a "natural order" of acquisition. This work fed directly into Krashen's Monitor Model and the Natural Order Hypothesis.
Morpheme accuracy studies remain a standard methodology in SLA research: researchers calculate the percentage of correct suppliance of target morphemes in obligatory contexts to measure acquisition stages.