Since引导的时间状语从句总以肯定的形式与完成时连用，但它所表示的意义却并非总是肯定的。就是说，这类从句在形式与内容上有时是统一的(肯定形式表示肯定内容)，有时又是对立的(肯定形式表示否定内容)。那么，规律何在? 一、since从句的谓语是终止性动词的一般过去时，从句的内容与形式是统一的，从句的意义是“自从……以来”。例如： It has rained agreat deal since you left．你走之后下了好多雨。 Since time-directed adverbial clauses are always used in affirmative form in conjunction with completion, the meaning it represents is not always affirmative. That is, such clauses are sometimes unified in form and content (affirmative forms express affirmative content), and sometimes contradictory (affirmative forms represent negative content). Then, what is the law? First, the predicate of the since clause is the general past of the terminating verb. The content and form of the clause are unified. The meaning of the clause is “since since...”. For example: It has rained agreat deal since you left. After you left, it was raining.