opportunity knocks but once 机不可失，时不再来

“机不可失，时不再来”，汉语成语，指“时机难得，必须抓紧，不可错过”，可以翻译为“Such an opportunity, once missed, will be forever gone”。还可以用“opportunity knocks but once”，意思是“great opportunities are typically only encountered once.”

例句：

机不可失，时不再来。哥们儿，如果你不去做，你会后悔的。

"Opportunity knocks but once, dude. You'll always regret it if you don't go for it."

机不可失，时不再来，这可能是你获得良好教育的唯一机会。

Opportunity knocks but once, and this may be your only chance to get a good education.