1.did 是do 的过去式；
2.had 是have 的过去式； 同时可以和其他动词的过去分词构成过去完成式,即had done.
He\'s doing very well at school.
他在学校里学习很不错。
Someone might find out, and that would never do.
也许有人会发现，那是绝对不行的。
What sort of person would do a thing like that?
什么人会干那样的事呢？
水利万物而不争,
而万物莫能与之争。
