This paper argues for a finer typology of temporal adverbial clauses (TACs) and provides an alternative way to understand the correlation between the internal syntax and external syntax of adverbial clauses in general. Presenting novel evidence from Cantonese, this paper shows that a temporal operator may merge within TP or at the highest CP to form TACs. The merging site not only determines whether operator movement is present, but also determines the attachment site of TACs to main clauses. In this way, the external syntax of adverbial clauses is reducible to their internal syntax.