英语中有少数介词可以兼作连词,比较常见的有after, before, since, till, until等。如:
After lunch it began to rain. 午饭后开始下雨了。(after为介词)
After you finish the letter show it to me.信写完后给我看看。(after为连词)
You must come home before dark. 你必须在天黑前回家。(before为介词)
I always bathe before I go to bed. 我睡觉前总要洗个澡。(before为连词)
I have been here since last week. 我从上周起一直在这里。(since为介词)
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。(since为连词)
英语中有些单词既是介词,又是连词。又如but:
Nobody knew her name but me. 除我以外,没有人知道她的名字。(摘自外研社、建宏《英汉多功能词典》,but 是介词)
No one heard it but me.(but 是介词) = No one but I heard it. (but 是连词)除了我以外,没有人听过这件事。(摘自外研社、建宏《英汉多功能词典》)
【说明】but ...在谓语动词前面时,but 是连词;but ...在谓语动词后面时,but 是介词。