1.表时间：since 自从.,接时间点,句子用完成时.如; They have been working on the project since the the end of last year. for+时间段,表示一个动作持续了多长时间,也用完成时.如：I have learned English for ten years. 2.表原因：since 从属连词,“既然”,如：Since everybody is here ,let's begin our lesson. for 并列连词,引导的并列句要放在后面.如:he didn't go to school,for he was ill.
