We claim that the root nonfinite verb phenomenon, or Optional Infinitive Stage, is a subcase of the larger phenomenon of Interface Delay, as a result of which distinct domains of cognition experience gradual development of their capacity to communicate with one another. Root nonfinite verbs, then, are the result of the delayed development of temporal anaphora – a construct that requires the interaction of syntax with the representation of interlocutors’ common ground perspectives. In the absence of adult-like interplay between syntax and common ground representations, children adopt a presupposition of familiarity. Delayed development of temporal anaphora is taken to be parallel to the delay in nominal anaphora manifested in the overuse of null subjects in child Spanish and the overuse of other definites, including overt tonic and clitic pronouns and definite articles, in an array of languages. The phenomenon in both typically- developing child Spanish and in the grammars of Spanish-speaking children with specific language impairment (SLI) is difficult, though not impossible, to detect in spontaneous production data because of the portmanteau nature of tense and agreement morphology, combined with the null subject nature of the language. These obstacles led early work to overestimate children’s competence in verb finiteness marking, but later work, especially using receptive tasks, has shown that child Spanish speakers produce, and accept as grammatical, an array of nonfinite forms including the bare stem (e.g. canta), the morphological infinitive (e.g. cantar) and the bare progressive participle (e.g. cantando). Internal validation of these findings comes from the fact that productive measures of nonfinite verb use correlate with receptive measures, downstream functions of tense marking (such as overt subject licensing) are significantly worse in Spanish-speaking children with SLI than in children without it and a discriminant function analysis based on elicited production measures of verb finiteness yields fair to good sensitivity and specificity (89%) for distinguishing Spanish-speaking children with SLI from those without. Further downstream evidence of optionality stemming indirectly from Interface Delay comes from optional child-particular grammatical constructions that depend on tense, including subject-auxiliary inversion and nominative Case marking in child English.