Hi Mehrdad When you say something couldn't be easier it means that it is the easiest way there is. It is impossible for it to be easier. When you say it couldn't be easy (or couldn't have been easy) it means that it must have been difficult. It was hard. So in a way they are opposites of one another. The first one means it was super easy and the second one means it must have been difficult. I hope this helps. Feel free to contact me if it is still not clear. Kind regards Nicci

برای مقایسه کردن دو یا چند چیز از Couldn't be easier و برای صحبت کردن راجب یه چیز از Couldn't be easy

The second one is often used as "That couldn't have been easy" when congratulating someone on a difficult feat / accomplishment. There is a similar phrase to the first one " I couldn't care less" which means that you care so little about something, that it is not possible to care any less.