I never would have guessed such a thing.

I would never have guessed such a thing.

两个句子都对，表达相同的意思。

这句话有两层含义：

1. 对过去情况的推测。

译为：我（过去，当时）根本不可能猜到会有这种事。

表推测的例句：

She would never have won a beauty contest. 她绝不会赢得选美比赛的。

He never would have made it very far. 他绝不会做得太离谱了。

Jordan never would have done that.  乔丹永远不会这么做。

2. 与过去不符的虚拟语气。

译为：（如果你不告诉我的话）我当时就绝对猜不到会有这种事。（实际已经猜到了）

虚拟语气的例句：

I would never have thought of it if you hadn't mentioned it. 如果不是你提起的话，我肯定想不到。

If I'd only known,I would never have gone away. 如果我知道，我绝不会离开你。

But for him, it would never have taken place. 如果没有他,这种事情绝对不会发生的。

Otherwise they would never have thought of doing this. 要不然，他们绝不会想到这样做。