When not in use, the fins are tucked into special grooves or depressions so that they lie flush with the body and do not break up its smooth contours.

当鱼不使用鱼鳍时，会将它们折叠起来，收回到特殊的沟槽或者凹陷部位；这样鱼鳍就可以与身体保持齐平，以维持它们平滑的外形。

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The table folds up when not in use.

这桌子不用时可以折叠起来。

《牛津词典》

Unplug laptop, desktop and cell phone power cords when not in use.

在不使用笔记本、台式机和手机时，把电源线拔掉。

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So you will use "it depends on" in a sentence when you're not sure of something is going to happen.

当你不确定某件事是否会发生时，你就可以使用“It depends on”句型。

It depends on 课堂 - SpeakingMax英语口语达人

When to use "the" in a sentence, when not to use "the", when to use "an" instead of "a" or "a" instead of "the".

在一个句子中，什么时候用“the”，什么时候不用“the”，什么时候用“an”代替“a”，什么时候用“a”代替“the”。

经常犯的失误 - SpeakingMax英语口语达人

Shades operating by wall-mounted controls disappear into a recessed ceiling pocket when not in use.

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Otherwise, the idea here is to park the mouse on its dock when not in use.

ENGADGET: Mobee's inductive Magic Charger for Apple's Magic Mouse really is magical

The LCD screen can also be flipped closed for protection when not in use.

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