since（自从；自.以来）的从句后面跟什么时态

since（自从；自.以来）的从句后面跟什么时态
急用 要单词 谢谢啊

When 当（在）.时；（=at the time that),可表示瞬间·时间段.与主从句所述动作·事情可同时,也可有先后例：It was snowing when we got to the airport.当我们到达机场时,天正下着雪.（同时发生） He went home when he had finished his homework.做完作业后他回家去了.（先后发生）While 正当（正在）.时；（指同时=①for as long as ②during the time that),用于同时进行的两延续性动词相伴随而发生,常对同类的两动作进行对比例：Please write while I read.(during the time that)我念的时候,请写下来.I am safe while I am here.(=for as long as)只要我在这儿,我就安全.Some students were reading while others were writing.有些学生在读,而有些学生在写.（进行对比）as 当...时；强调at the same time,①不指先后,而指并相发生,尤指短动词或事件同时发生例：As I looked,some one came near.正当我看的时候,有个人走上前来.②as 还可说明两种正在发展或变化的情况例：As I get older I get more optimistic.我越活越乐观.as,when或while可引导时间较长的“背景”情况例：As(When或,While) I was walking down the street,I noticed a police car in front of Number 37.我顺着马路往前走,发现37号门前停着一辆警车.while 从句动词须能延续在我们出门不在的时候,有人破门而入.误：Somebody broke in while we went out.正：Somebody broke in while we were out.While从句中要用延续性动词before 是指从过去某一时间算起的一段时间以前,意思是“在那时以前”·“更早的时候”,动词常用过去完成时,特别是在间接引语中用的多例：I visited him three days ago,but his neighbour said he had gone to London two days before.我3天前去拜访他,可是他的邻居说早在两天以前他就到伦敦去了.since ①【引起时间状语从句,从句中常用过去时,主句用一般时态或完成时】自从...以来；...以后②【引起原因状语从句,通常置于句首】既然,因为 希望能帮到你%D%A