must

Passengers must wear seatbelts at all times during the flight.

(Chinese) 乘客在飞行过程中必须全程系好安全带。

You must finish your homework before you can play video games.

(Chinese) 你必须先完成作业才能玩电子游戏。

It's already dark outside, so it must be after 8 PM.

(Chinese) 外面已经黑了，所以肯定已经过了晚上八点。

He must be exhausted after working a double shift at the factory.

(Chinese) 在工厂连值两个班后，他一定筋疲力尽了。