A commonly provided definition for sentence types in traditional Arabic grammar (إعراب) is that nominal sentences (الجملة الإسمية) begin with nominals (nouns, pronouns, etc.) and verbal sentences (الجملة الفعلية) begin with verbs. However, this definition fails to cover more complex sentences such as those with verbs following negative particles, or for sentences headed by (كان واخواتها) or (ان واخواتها).
A more functional definition is that a nominal sentence contains the roles of subject (مبتدأ) and predicate (خبر), whereas a verbal sentence contains the roles of verb (فعل) and subject (فاعل). From this definition, it follows that a sentence headed by (كان واخواتها) or (ان واخواتها) is originally a nominal sentence, and remains a nominal sentence after the introduction of (كان), since this verb is considered to be incomplete (ناقص) in this usage.