One of the challenges confronted by language learners is to master the interpretation of sentences with multiple logical operators (e.g., nominal quantifiers, modals, negation), where different interpretations depend on different scope assign- ments. Five-year-old children have been found to access some readings of potentially ambiguous sentences much less than adults do (Lidz and Musolino, Lang Acquis 13(2):73–102, 2006; Musolino, Universal Grammar and the acquisition of seman- tic knowledge, 1998; Musolino and Lidz, Lang Acquis 11(4):277–291, 2003, among many others). Recently, Gualmini et al. (Nat Lang Semant 16:205–237, 2008) have shown that, by careful contextual manipulation, it is possible to evoke some of the putatively unavailable interpretations from young children. Their proposal is quite gen- eral, but the focus of their work was on sentences involving nominal quantifiers and negation. The present paper extends this investigation to sentences with modal expres- sions. The results of our two experimental studies reveal that, in potentially ambiguous sentences with modal expressions, the kinds of contextual manipulations introduced by Gualmini and colleagues do not suffice to explain children’s initial scope interpre- tations. In response to the recalcitrant data, we propose a new three-stage model of the acquisition of scope relations. Most importantly, at the initial stage, child grammars make available only one interpretation of negative sentences with modal expressions. We call this the Unique Scope Assignment (USA) stage.