He overbalanced and fell head first.

他失去了平衡，头部先着了地。

《柯林斯英汉双解大词典》

He overbalanced and fell into the water.

他失去了平衡，落入水中。

《牛津词典》

But it suddenly overbalanced and slid back into the water.

但它突然失去了平衡，又跌到水中。

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But it suddenly overbalanced and slid back into the water.

但它突然失去了平衡，又跌到水中。

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