6.since与现在完成(进行)时

since可当作介词、连词，间或也可以当作副词(参见状语从句)。注意观察下面句子中since的词性:

Since the mid-1980s growing numbers of Asians have settled in New Zealand．(人教大纲版高一下第41页)

从二十世纪八十年代中期开始，来新西兰定居的亚洲人越来越多。

Readers have wondered about the character of Captain Ne mo ever since the book was published．(人教大纲版高二下第12页)

自从这本书出版后，读者就对尼莫舰长的性格有很多疑惑。

The first edition was published in 1955 and has been a best seller ever since．(人教大纲版高三第3页)

1955年出了第一版(《吉尼斯世界纪录大全》)，以后就一直是畅销书。

Since my coming to London I have been working hard．

自我来伦敦以来，我一直很努力地工作。

I have been looking forward to seeing you since I saw you last．

自上次见到你以来，我就一直盼望着再次看到你。

从上述诸例可以看出:since作介词用时后面跟表示过去的点时间，作连词用时状语从句用一般过去时，主句则用现在完成(进行)时。

必须注意:since状语从句的动作是终止的，如果这时的动词是非延续性动词(上述诸例均如此)，其含义与中文是相同的。但如果动词是延续性的，就必须考虑这个动作的终止。

It is six years since he studied English．

他不学英语(终止学英语)已有六年了。

I have met him twice since he stayed in Shanghai．

自他离开上海(不住在上海)以来，我见过他两次。

He has written to me frequently since I was ill．

自我病愈以来，他常给我写信。

如果确实要表达与中文相同的含义，则可以在since从句中使用现在完成(进行)时。

It is six years since he has studied English．

他学英语已有6年了。

I have met him twice since he has been staying in Shanghai．

自他住在上海以来，我见过他两次。

如果since状语从句的动词是非延续性动词，用一般过去时和现在完成(进行)时，意义上没有差别。

How long is it since you heard/have heard from Jane?

自你收到简的来信有多久?

He has been in good health since he joined/has joined the army．自他入伍以来，身体一直很好。