What is the nuance difference between "You can't be serious" and "You are not serious"?
Native speaker’s answer
Rebecca
"You can't be serious" and "You are not serious" have different meanings. "You can't be serious" means "What you're saying is ridiculous" and shows disbelief about what someone is saying. "You are not serious" is accusing someone of not being serious. "You are not serious" by itself is not common to say. The more common phrase is "You are not serious about (something)". Ex: A: I'm going to go skydiving this weekend even though I'm afraid of heights. B: You can't be serious. Ex: I can tell you are not serious about school because you receive bad grades.