This study examines heritage speakers' knowledge of Standard Arabic (SA) and compares their patterns of SA acquisition to those of learners of SA as second/foreign language (L2). In addition, the study examines the influence of previously acquired language varieties, including Colloquial Arabic (QA), on SA acquisition. To this end, the study compares 35 heritage speakers, 28 L2 learners, and 16 controls with respect to sentential negation, an area where SA and QA diverge significantly. The participants completed five oral tasks targeting negation of eight different clause types. The findings showed that L2 learners and heritage speakers performed comparably, encountered similar difficulties, and produced similar patterns of errors. However, whereas L2 learners did not display clear transfer effects from L1 (English), heritage speakers showed both positive and negative influence of L1 (QA). The results shed light on the dynamics of the interaction between the spoken heritage languages and their written standard counterparts with specific focus on diglossic contexts.