机不可失，时不再来。

It was an opportunity not to be missed.

《牛津词典》

与此同时，正在维也纳美国能源部部长朱棣文也表示哥本哈根会议并不是“机不可失，时不再来”，以后人们还可以有机会对温室气体的排放进行规定。

Meanwhile, Energy Secretary Steven Chu, traveling in Vienna, said Copenhagen is not a "now or never" chance to scale back global greenhouse gas emissions.

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可能你会想“机不可失，时不再来”，既然已经失去了，怎么可能再追得回来？

Perhaps you may want to "now or never", since it has been lost, how could catching back?

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