It's used as an adjective when you are frustrated or when you are angry at something.

当您感到沮丧或对某事生气时，它用作形容词。

" I can't believe it" is something you say when you're very angry at something, when someone has disappointed you, has done something wrong.

“我简直不敢相信” 是当你对某事非常生气， 当有人让你失望， 做错事时，你会说的话。