It has been commonly held that idioms have a number of unpredictable irregularities. However, I claim that there exist systematic semantic regularities in certain verbal idioms that are commonly manifested in the semantic structure of causative verbs. First, through the investigation of the causative/inchoative alternation, I propose the general semantic principle ([x P1 [y P2 (z)]]→[y P2 (z)]) which states that the embedding semantic predicate P1 is removable from the bipartite semantic structure. Then, I discuss six classes of verbal idioms with the bipartite semantic structure, and argue that the semantic composition of these idiom classes is also governed by the same general principle by demonstrating that each class of these verbal idioms has its corresponding class of verbal idioms of the form [y P2 (z)].