dont的过去式是什么

例句：

I know him, but we dont frequently see each other.

我认识他，但我俩并不常来常往。

Dont' think it will all be that easy.

不要把这事看得那么容易。

Don't speak in a loud voice!; Dont't make noise!

不要大声喧哗！

B: I dont think it fun to travel in big cities.

B：我认为在大城市旅行不好玩。