1.表示“当……时候”或“每当”的从属连词：when，while，as，whenever
Don’t talk while you’re eating.吃饭时不要说话。
Vegetables are best when they are fresh.蔬菜新鲜时最好吃。
He came just as I was leaving.我正要走时他来了。
I’d like to see you whenever it’s convenient.一有方便的时间，我就去看你。
2.表示“在……之前（或之后）”的从属连词：before，after
Try to finish your work before you leave.离开前设法把工作做完。
After we have finished tea，we will sit on the grass.喝完茶之后我们将坐在草地上。
3.表示“自从”或“直到”的从属连词：since，until，till
She’s been playing tennis since she was eight.她从八岁起就打网球了。
Hold on until I fetch help.坚持一下，等我找人来帮忙。