助动词 do 的用法

1) 构成疑问句和否定句
* Do you smoke? 你抽烟吗？
* I didn't know my own strength. 我不知道自己的力量。
* I don't know what to think. 我不知道该想些什么。
* Why didn't you tell me? 你为什么不告诉我呢？

2) 加在动词前表示强调
* He did finish his homework. 他的确完成作业了。
* Do sit down. 务必请坐。
* She does work at home. 她的确在家工作。
* Do be careful when you cross the street. 过马路时，千万要小心啊。
* I do hope you will stay for lunch. 我希望你能留下来吃午饭。

3) 表示刚提到过的动作， 以避免重复。
* “She likes cakes.” “She really does.”“她喜欢蛋糕”“她的确喜欢”
* He doesn't smoke but I do. 他不抽烟，但我抽。
* You like football but your brother doesn't. 你喜欢足球，但你的弟弟不喜欢。
* You didn't write to him, did you? 你没有写信给他，是吗？
* “Does he like coffee?” “No, he doesn't” “他喜欢咖啡吗？”“不，他不喜欢”

[注] do 也可作为实义动词用, 如:
* Have you done your homework? 你做完作业了吗?
* What are they doing? 他们在做什么?
* He's doing well. 他做得很好。
* These shoes won't do for climbing. 这些鞋不适合於登山。