一旦你开始看它上面的视频，就很难停下来。

It's difficult to stop once you start watching its videos.

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火车一旦开动就没办法让它停下来。

Once the train is moving, there's no way to stop it.

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成千上万的英国妇女可能正在按自己的全科医生\家庭医生开据的处方服用抗抑郁药，而不知道一旦服用该药便很难把它停下来，更不知道服用该药可能会导致新生儿先天缺陷。

Thousands of women in the UK may be taking antidepressants prescribed by their GPs without knowing that the pills, which are hard to stop taking, could cause birth defects in unborn children.

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