since

1.注意下面各句里都用完成时态：

• He returned a year ago, and has since lived with us.
• He has lived with us since his return.
• He has lived with us since he returned.
• He had returned a year before, and had since lived with us.
• He had lived with us since his return.
• He had lived with us since he (had) returned.

第一句和第四句里的since是副词，第二句和第五句里的since是介词，第三句和第六句里的since是连接词。第六句里用returned比用had returned普通。

下面各句里并不用完成时态：

• He returned a year ago, and his health is much improved since.
• He writes the best English since Dr Johnson.
• Mr Lewis, living with us since he returned, is now a well-known author.

但虽然不用完成时态，却有完成时态的意味：第一句里的is much improved有is much improved because it has been much improved的意思，第二句里的writes the best English有writes the best (English) of the English that has ever been written的意思，第三句里的living有living as he has lived的意思。

It is若干时间since…比It has been若干时间since…普通得多，如通常说It is a year since his return和It is a year since he returned，很少说It has been a year since his return和It has been a year since he returned。

2.He has lived with us since he returned里的returned属于过去时态，当然不可改作returns。He has lived with us ever since he can remember里却用属于现在时态的can，但he can remember有the earliest time that he can remember的意思。

3.He has lived with us since he returned里的returned属于过去时态，不可改作has returned。在since所引导的从句里不用过去时态而用现在完成时态是英美人常有的错误。

比较下面三句：

• I have lived in this house since I came to Shanghai.
• I have lived in this house since I have been in Shanghai.
• I have lived in this house while I have been in Shanghai.

第一句当然不错。第二句不及第一句合乎语法，但have been用在since引导的从句里很普通，比其他属于现在完成时态的动词普通得多，可以算不错。第三句当然不错，但不及第二句普通。

下面两句意思不同：

• He has written to me frequently since I was ill. （自从我病愈以来，他屡次写信给我。）
• He has written to me frequently since I have been ill. （自从我生病以来，他屡次写信给我。）

4.since（介词）的宾语该是一点时间（a point of time），不是一段时间（a period of time），如可以说I have lived here since January，不可说I have lived here since five months，该改作I have lived here for five months。

5.不可说“若干时间since...”，如不可说I have lived here five months since my return或I have lived here five months since I returned，该改作I have lived here since my return five months ago或I have lived here since I returned five months ago。

6.since往往用作ago。例如：I returned a year since.

比较下面三句：

• It is a year since that I returned.
• It was a year since that I returned.
• It is a year since I returned.

第一句和第二句里的since都作ago解，第一句着重“到现在已有一年”的意思（参见ago 3），第三句里的since是连接词。

7.since（介词）往往把动名词做它的宾语。例如：I have had three letters from him since writing last. 和He has not done it again since being warned. 注意writing的动作是属于I的，being warned的情形是属于He的。不可说Three letters have come since writing last和It has not been done by him again since being warned。

8.since（连接词）往往用作“因为”解。例如：Since you do not understand, I will explain again. 和He did not come, since he was very busy.

Since you do not understand, therefore I will explain也可以说，但通常不用therefore。注意不可说Since you do not understand, so I will explain again。

9.Well, since you ask me, I never was in love with your sister, though I found that out only when I fell in love with you里的since you ask me并不真正表示原因，那主句所说的是客观事实，并不因为since you ask me。这种用法的since跟if 6里的if相仿。

10.since（因为）语气比较轻，所以不可说It is since you do not understand that I will explain again，该把since改作because。

11.long since（副词）作“久已”解，通常用在完成时态里，有时也可以用在过去时态里，如Bows and arrows have long since been out of use, All has long since vanished and become a memory和Their crews were long since asleep.