I think it's hard to believe. 我觉得这很难相信。

It's too good to be true. 哪有这等好事？

I don't buy your story. 我才不信你的鬼话呢。

Are you joking/kidding me? 你在逗我呢吧？

I am doubtful about that 我对此表示怀疑。

Don't expect me to believe this. 别指望我相信这事。

It can't be ture. 不可能是真的。

How is that possible? 那怎么可能呢？

I never take him too seriously. 我从来不把他的话当真。

I feel puzzled. 我感到很茫然。