In the English "tough" construction (TC), knowledge of "tough" movement is necessary for target performance (the object-interpretation only; e.g. "John is easy to see" e). The acquisition of the English TC raises a learnability problem for first-language (L1) Korean learners of English as a second language (L2): (1) Korean has no "tough" movement; (2) no input dictates that the 'subject interpretation' is disallowed in the English TC; and (3) no classroom instruction covers the English TC. According to the Fundamental Difference Hypothesis, L2 children -- but not L2 adults -- can overcome this learnability problem. L1-Korean adult (n = 49) and child (n = 30) L2 learners' (L2ers') knowledge of the English TC was assessed via a truth-value judgment task manipulating (1) verb transitivity to make the infinitival object gap more vs. less salient and (2) context to avoid vs. strengthen bias toward the (erroneous) subject interpretation. Notably, some high-proficiency adult L2ers showed significantly above-chance performance, despite the error-inducing manipulations, suggesting that adult L2ers can overcome the learnability problem.