英语什么介词可当作连词用

也就是说它既是介词，又是连词，譬如 after, before，还有什么也是如此的？

英语中有少数介词可以兼作连词，经常见到的有after, before, since, till, until等。如：

After lunch it began to rain. 午饭后开始下雨了。

After you finish the letter show it to me．信写完后给我看看。

You must come home before dark. 你需要在天黑前回家。

I always bathe before I go to bed. 我睡觉前总要洗个澡。

I have been here since last week. 我从上周起一直在这里。

She has had another baby since I saw her last. 从上次我见到她以来，她又生了一个孩子。