must have been的意思是什么

造句：

1.Judging from his listless appearance, he must have played video games again last night.

看他那没精打采的样子，一定是昨天夜里又玩电子游戏了。

2.Great discoverers are not always great men。

伟大的发现者并不一定是伟人。

3.I'm sure it would be the best story in the world.

我相信，那一定是世界上最美好的故事。