The microwave beeps to let you know when it has finished.

微波炉烹饪完毕时会发出哔哔声提醒你。

《牛津词典》

It's a bit inconsiderate of her not to let you know when she expects to arrive.

她不让你知道她什么时候会到，这样做有些不体谅人。

《柯林斯英汉双解大词典》

You should invite your readers to let you know when they spot an error.

当你的读者改正你的错误时，你应该让他通知你。

youdao

As well as keeping track of its contents, the bag also lights up in the dark and will soon be able to check weather forecasts to let you know when to pack an umbrella.

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Take as much time as you want to think it through, and let me know when you want to start.

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Parking sensors let you know when to brake, return the vehicle to drive and pull forward.

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