1、since用作介词和副词时，其句中谓语动词一般用完成时（包括现在完成时和过去完成时）；用作连接词时，通常从句用一般过去时，主句用现在完成时，这时since-从句中的动词通常为非持续性动词，表示主句中现在完成式动作的起始点。如
I want to see how much he has changed since I saw him last. 我想看看自从上次见他后。他变化有多大。（连词）
I've known him since our schooldays. 从小学我就认识他。（介词）
I met her in 2000 and haven't seen her since.我于2000年遇见她，从此再没有见过（副词）
2、当since-从句中的动词是持续性动词或状态动词的一般过去时时，从句不再表示完成式动作的起始点，而表示动作的完成或结束。如
The house has been in bad repair since they lived in it. (=...since they moved out of it.) 自从他们搬出以后，这房子一直没有很好的维修。
He has had several jobs since he was a student at Yale.(=...since he graduated from Yale.) 自从耶鲁大学毕业后，他已经干过好几种工作了。
在这种句型中，要特别注意since-从句中动词用一般过去时和现在完成时两种不同时态时所表达的不同意义。如
It's /has been five years since they lived here. 他们离开这里已经5年了。
It's /has been five years since they have lived here. 他们住在这里已经5年了。
3、since-从句一般不用否定结构。如不说
It's twenty years since I didn't see him. 而应说 It's twenty years since I saw him last.
4、since用作连接词，主从句谓语动词的几种形式：
（1）主句用一般现在时，从句用一般过去时
I feel much better since I took the medicine. 服过药后，我感觉好多了。
当主句表示“已经有多少时间”时，动词应用完成式。但在口语中，用一般现在时较普遍
How long it is since she left Beijing? 她离开北京多久了？
（2）主句用完成时，从句用一般现在时
I have had such impressions since I can remember. 从会记事起，我就有这样的印象。
（3）主从句都用完成时，这时since-从句通常用持续性动词或状态动词表示动作或状态延续到说话时刻
I haven't seen you since I've been back. 从我回来至今还没有见过你。
（4）主句用一般现在时，从句用完成时
It's some considerable time since I have spoken to you on these matters. 主从我对你讲了这些问题，已有相当一段时间了。
（5）主句用完成进行时，从句用一般过去时
I have been looking forward to meeting you ever since Arther told me. 自从阿瑟告诉我，我一直在盼望着见你。
（6）主句用完成进行时，从句用现在完成时
I have been hearing a lot of things about you since I have been back in this country. 自从我回到本国，我不断听到有关你的一些事情。
