cloud

n. 云,忧色,云状的烟
v. 以云遮敝,笼罩

The sad news of her father's death was the only cloud on an otherwise happy year.

她父亲的死讯是她愉快的一年中唯一伤心的事。

When there are black clouds you can tell it's going to rain.

有乌云的时候，就知道快下雨了。