This paper addresses the question of how learners break into a novel morpho-syntactic system, extract elements of this new system from the input they receive, process them, and begin to acquire the new system. The data for this project were collected as part of a large European project (VILLA – Varieties of Initial Learners in Language Acquisition) comparing the processes of perception, comprehension, and production during the acquisition of a novel target language (Polish) in adults of different source languages within the first hours of instruction under controlled input conditions. Two experiments, a grammaticality judgment task and an oral question-answer task, were conducted longitudinally to investigate learners’ perception and use of Polish nominal morphology in two groups of adults (French and German native speakers) after 4.5 hours and after 10.5 hours of instruction. In addition to contributing new insights into the role of the source language in the initial stages of acquisition, results speak to the influence of overall exposure to the input, and reveal interesting interactions between factors such as the frequency of a lexical item in the input and its transparency relative to the learner’s source language.