The correct use of morphological suffixes in obligatory contexts reflects linguistic knowledge and competence of speakers. Grammatical knowledge is acquired during a child’s period of primary language acquisition, and may be partial or incomplete due to normal linguistic variation found during acquisition, due to a child’s level of progression through typical chronological development, or due to the presence of language disorders, like specific language impairment (SLI). In the current study, we ask whether characteristics of verbs make it more or less likely that children will correctly use an inflectional morpheme. The morphemes of interest in the current study were third person singular –s (3S) and past tense –ed (ED). Data for analysis were taken from a database of spontaneous language samples collected from 40 children (20 with SLI and 20 developing typically; Hoover, Storkel, & Rice, 2012). Spontaneous language samples were analyzed for the presence or absence of each morpheme in obligatory contexts. For each word item, the uninflected base word was additionally analyzed for a number of phonological and lexical variables. After comparing children with SLI to typically developing peers group differences emerged with respect to the effect of phonological and lexical variables. Moreover, different variables were determined to predict the 3S and ED morphemes. The results are discussed highlighting relevant theoretical and clinical implications.