对这几种语态问题 甚是迷茫！请详解其区别 谢谢

I have done it. 我已经做过（做完）了。
I have been doing it. 我（从之前到现在）一直在做。

I had done it跟I had been doing it.不能直接做为一句话来用的，过去完成时表过去的过去。

要有一个过去的时间点，才能提过去完成时。
如：

I told him that I had done it. 我告诉他我做完了。（在我告诉他的时候（过去时），我已经做完了。）
I dreamt that I had been doing it. (我梦到（过去时）我此前一直在做这件事。)