The morphology-syntax interface concerns how morphological and syntactic processes interact. Is morphology derived from syntax (words built by phrase-building operations), or separate? Modern approaches like Distributed Morphology argue morphosyntactic features are assigned syntactically, but actual word forms are determined at the phonological interface, unifying morphology and syntax under one derivational system.
Your prerequisite in morphological structure gave you the tools to analyze how morphemes combine within words — affixes, roots, inflectional versus derivational morphology, paradigm structure. Your acquaintance with syntactic movement introduced hierarchical phrase-building and displacement operations. The morphology-syntax interface asks the question that bridges these two areas: are word-building and phrase-building fundamentally the same kind of operation, or are they separate cognitive modules that merely interact at their boundaries? The answer has consequences for the entire architecture of the grammar.
The strongest evidence for tight interaction between morphology and syntax comes from agreement: the morphological form of a verb reflects its syntactic relationship to a subject. In Latin or Spanish, verbal endings change according to the person and number of the subject — morphological realization that tracks syntactic structure transparently. If morphology were truly separate from syntax, this correspondence would have to be stipulated, not explained. Head movement provides another piece of evidence: in many languages, a verb moves from its base position in the verb phrase to a higher functional position (Tense, Agreement), and this movement is reflected morphologically — V2 in Germanic languages, V-to-T movement in French. The movement and the morphology are two sides of the same coin.
The dominant theoretical divide concerns where complex words are built. Lexicalism holds that morphologically complex words are assembled in a pre-syntactic module — the lexicon — and syntax sees only the completed words it inserts. The Weak Lexicalist Hypothesis restricts this to derivational morphology (word-formation like *un-happi-ness*), allowing syntax to handle inflection. The Strong Lexicalist Hypothesis extends it to all morphology. Against this, non-lexicalist approaches argue that the apparent distinction between word-building and sentence-building dissolves under scrutiny — both involve hierarchical feature-structure building, differing in scale but not in kind. Distributed Morphology, your next topic, is the most fully developed version of this position.
The interface problem resists easy resolution because the same surface patterns can be described in multiple theoretical vocabularies. A language where verbs agree with subjects can be analyzed as syntax driving agreement (non-lexicalist) or as the lexicon inserting agreement-marked forms after syntactic derivation (Lexicalist). The choice between these analyses depends on subtle empirical differences: mismatches between morphological and syntactic structure, the behavior of clitics, the distribution of phrasal idioms, the properties of incorporation. Mastering the interface means learning to see what each theory predicts that the other cannot handle — which is the method of theoretical linguistics at its core.