do的過去式是什么？

　　例句：

　　What in the world did they think they were doing?

　　他們到底認為自己在做什么？

　　I could do it now, if you like.

　　如果你愿意的話，我現在就可以做這事。

　　What is he doing calling at this time of night?

　　他干嗎這么晚了還打電話？