<p>Language teachers need to be aware of the factors that influence their students' comprehension of second language words. This study explores: (a) whether learner knowledge about base words and knowledge about their corresponding derivational forms (DF) is significantly different: and (b) if the presence or absence of context influences the relative difficulty of related DF. A meaning-recall test format was used to measure the knowledge about various forms of five high frequency base words (<i>accept, help, move, operate</i> and <i>read</i>) among a cohort of 150 tertiary-level Japanese English as a foreign language learners. Two hypotheses are tested: (a) that there would be a strong interdependence between learner knowledge about base words and DF (with single and multiple affixes) from the same word family" and (b) that DF presented in context would be easier for learners than those presented without context. Results suggest that for these target words among this cohort, each hypothesis should be rejected. Specific pedagogical recommendations for the development of knowledge about DF among L2 learners are provided. The discussion addresses the importance of classroom teachers refining the heuristics used to guide pedagogy and that the strategic collection of learner data via formative assessment is crucial to this process.</p>