What you are saying can't/couldn't be true.

[hirashin]

Dear native English speakers,
Do both the sentences sound natural? If so, what's the difference?
(a) What you are saying can't be true.
(b) What you are saying couldn't be true.

Hirashin

 

[mdchachi]

Both sound natural (textbook English) and no difference in meaning to me.

 

[Buntaro]

Hirashin,

Both sentences are correct. The meanings are different. The use of "couldn't" is subjunctive mood (仮定法). Are you familiar with the uses of subjunctive mood? For example, what is the difference between:

(c) If I go to Hawaii, how much will it cost?
(d) If I went to Hawaii, how much would it cost?