如果改成i'm really can't do that.还对么?

I really cannot do that 或者是 i really can't do that 都行。

不行，一句话只能有一个主语一个谓语，这句话说成I can't do that,really.会好一些~~

那就不对了，一个句子中只能有一个动词作谓语的。
这个句子中的 am 和 do 都是谓语动词，所以就不对了。
如果想用 am 做谓语的话，可以改成I'm really sorry that I can't do that.

不对
有两个动词
am 和 can
应该是
I can't really do that

不对
I really can not do that.