The acquisition of morpheme-structure constraints by children is discussed. The focus is a subset of verbs in modern Hebrew and the language-specific knowledge that children acquire of what constitutes a possible verb in their language, from the point of view of both internal form and of categorical appropriateness for naming a certain semantic content or transitive relation. The application of two complementary processes, root extraction and pattern assignment, by 60 Hebrew-speaking children and 12 adults was studied. Subjects were asked to interpret and produce innovative verbs based on familiar nouns and adjectives. The three main findings include the following: (1) children can perform root extraction from as young as age three, and do better at identifying consonantal roots when they are presented with novel verbs for comprehension than in producing novel verbs by extracting roots from nouns or adjectives they know; (2) when children produce new verbs, their innovations conform closely to the grammatical structure of the standard morphological patterns used for constructing verbs in Hebrew; and (3) all child subjects, aged 3-9, overwhelmingly favor the verb pattern preferred for denominal verb-formation in current Hebrew, even though other patterns are equally available in the established lexicon and in the children's own speech. (MSE)