We must sleep when it is dark.

当它是黑暗的时，我们必须睡觉。

youdao

When it is dark enough, you can see the stars.

只有夜色足够黑的时候，才能看得见星星满天。

youdao

I mean I had been in darkness before; however, even when it is dark I could at least see something.

自己以前也曾在黑暗中待过，可是那至少也能看到一些东西。

youdao

The bins "talk" only during daylight hours to prevent people from getting a fright when it is dark.

CNN: Talking trash cans keep Berlin clean

The council said the thermal imaging surveys needed to be carried out in the evening when it is dark and the home is being heated.

BBC: Clackmannanshire Council to use thermal imaging surveys

Fingerprints aren't as noticeable when the device is showing bright, colorful images, but become starkly visible when the iPad is turned off or when it is playing a dark scene in a movie.

WSJ: Greasy Smudges Frustrate iPad Owners