when doing,when done, when to do的用法.

几个例子：

when doing his homework,he heard someone singing
当正在做作业的时候，他听到有人唱歌。
When (you are) finished with your work,call me
当做完了作业，给我打电话。
I don't know when to go
不知道什么时候走。

这个很简单 ~ 就是时态的问题么 ~·！
doing 不是正在的意思~！ 是现在进行时用多将来进行时！ 是以后
done就是 什么时候做的 ！表示过去了 ~
to do 就是一般现在时了~！·！~

doing 表主动进行
done表被动完成
to do表将来

doing 表主动进行
done表被动完成
to do表目的结果

when doing--something else's happening/ concurrently.
e.g. When doing this question, you have to look at the previous answer too!

when done--to be finished off / if...then kind of situation
e.g. Tell me when you are done with that, i will give you more things to do.

when to do--to be finished/not sure about the timing of incident
e.g. Tell me when to do that renovation?
I don't know when to do the next exercise?