表示判断的情态动词通常是may、must、can’t

may 表示不确切推测或可能，相当于I am not sure...
The man may be in that room. 那人有可能在那个房间里。

must 表示肯定的判断，相当于I am sure...
The man must be at home. 那人肯定在家。

can 表示人所共知的客观可能性或听话人已知的客观事实。
If you are brave enough, all difficulties can be overcome.
如果你不畏艰险，所有的困难都一定会克服。
Correct ideas can be transformed into a material force that changes the world.
正确的理念一定能够转化为改变世界的物质力量。
I hear that you are going to Thailand. It can be dangerous to be there, I think.
听说你要去泰国，我看到那儿去挺危险的。

can’t 表示否定的可能性。
The boy is honest. He can't lie.
那男孩很诚实。他不可能说谎。
There is no light in her room. She can't be at home now.
房间里没有灯光。她现在不可能在家里。