While L2 learners are less sensitive than native speakers to morphological structure in general (Clahsen & Felser, 2006; Jiang, 2007; Neubauer & Clahsen, 2009), researchers disagree about the roles different features of morphological systems play in determining the timecourse and accuracy of their acquisition by L2 learners. Some studies suggest that L2 learners process derivational morphemes in a more native-like manner than inflectional ones (Silva & Clahsen, 2008; Kirkici & Clahsen, 2013). Other research demonstrates accurate acquisition of L2 inflectional morphology as well (Gor & Jackson, 2013; Hopp, 2003; Jackson, 2008; Sagarra & Herschensohn, 2010). To date, few studies have directly compared L2 acquisition of inflectional and derivational morphology (Silva & Clahsen, 2008; Kirkici & Clahsen, 2013). Arabic verbs exhibit a system of derivational morphology whose function in constraining event structures and theta roles allows for comparably direct comparison with inflectional morphemes at the sentence level.

Forty-four L2 learners and thirty-three native speakers of Arabic participated in the current study, which used three behavioral tasks: a primed lexical decision task, an acceptability judgment task, and a self-paced reading task, to triangulate a picture of L1 and L2 Arabic learners' commands of derivational and inflectional morphology at the lexical and sentential levels. Results of the lexical decision and self-paced reading tasks indicated that both L2 learners and native speakers alike made use of Arabic derivational and inflectional morphological structure during lexical access and sentence processing. However, the acceptability judgment task found that L2 learners made far more accurate judgments about Arabic inflectional errors than about derivational errors. By contrast, native speakers made accurate judgments about both kinds of morphological errors. Thus, L2 learners' behavior regarding Arabic inflectional morphology was at least as native-like as their behavior regarding derivational morphology, if not more so, across tasks. This pattern of results accords with previous research that found accurate processing of inflectional morphology in proficient L2 learners. It also adds to a growing body of research suggesting that the distinction between derivational and inflectional morphology in Semitic languages may be more graded than it is in Indo-European languages (Boudelaa & Marslen-Wilson, 2000; Frost, Forster, Deutsch, 1997).