Opportunity knocks but once.

良机难再。

《新英汉大辞典》

Opportunity knocks only once.

机会只有一次。

youdao

You May not Get Another Chance - Sometimes opportunity only knocks once.

你也许没有第二次机会——有的机会只会降临一次。

youdao

Opportunity knocks but once, so we must make the most of it.

机不可失，失不再来，所以一定要把握时机。

youdao

Opportunity knocks but once, so we must make the most of it.

机不可失，失不再来，所以一定要把握时机。

youdao