15. First Language Acquisition Laura Wagner 1. Introduction As this volume shows, the syntax and semantics of tense and aspect are complex, and vary in interesting ways across languages. From the perspective of a child who has to learn the particular temporal system of her language, the complexity and variability pose very real learning problems. Children face several critical challenges in acquiring tense and aspect. One challenge concerns the variability in linguistic marking, which encompasses a wide range. Some languages mark only grammatical aspect while others mark only tense (compare Mandarin and Modern Hebrew); when a language does grammatically encode these elements, it can do so via many methods, including modifications to the verb stem (as in Russian), separate particles (as in Mandarin), verb morphology that combines aspectual and tense information together (as in French), and implicit signaling using case marking on nouns (as in Finnish). Thus, simply finding the relevant morpho-syntactic elements that express temporality is a non-trivial task for the learner. Another challenge concerns the complexity of the temporal semantic system. Tense and aspect consist of several overlapping and related elements that interact with one another; moreover, the specific types of interaction that a language allows depend on the specific instantiation of the semantic elements in that language. For example, the combination of imperfective aspect with stative predicates is allowed in some languages but not others. In particular, when the imperfective form in a language has a progressive flavor to it, languages tend to disallow it (as in English). Thus, children must learn fine-grained semantic distinctions so that they can create the right system across their temporal elements. A final challenge for children stems from their cognitive immaturity: children must be able to understand the concepts that underlie temporal semantics, as well as be able to identify situations in the world