Melanin darkens the skin and is the cause of suntan .

黑色素使皮肤变黑是人被晒黑的原因。

As the sun sets , the sky darkens and the moon and stars appear .

随着太阳落下，天空逐渐变黑，而月亮和星星开始出现。

But this color darkens when exposed to the sun .

但这种颜色暴露在阳光下将会变暗。

And if house prices fall , the picture darkens .

而且如果房价下跌的话，形式会一片黑暗。

Hatred darkens life ; love illuminates it .

仇恨使生命黑暗，爱却能照亮生命。