The purpose of this study was to determine whether it is possible to distinguish between "difficult" and "easy" constructions for second language (L2) learners by examining characteristics of the structures as they occur in aural input. In a multidimensional analysis of 3 English structures with different acquisition profiles--the simple past, possessive determiners "his/her", and the progressive aspect--we examined the phonological, morphosyntactic, and lexicosemantic characteristics of the forms as they occurred in a 110,000-word corpus of instructional talk to L2 learners. We analyzed the type/token distributions of the forms, their lexical properties, and their perceptual salience. Our findings revealed key input factors that distinguished between the early-acquired progressive, on the one hand, and the later-acquired past and "his/her" determiners, on the other hand. These results lend support to theoretical accounts of the input-acquisition relationship and also generate hypotheses for manipulating instructional input to increase the salience of opaque constructions.