Opportunity knocks at the door only once.

机不可失,时不再来.

《简明英汉词典》

Opportunity knocks but once.

良机难再.

《现代汉英综合大词典》

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.

互联网

If opportunity knocks, I will take it.

如果机会来临, 我就会抓住它.

互联网

Opportunity knocks often -- how are you home?

机遇经常来敲 你家 的门,但是你是否总是在家?

互联网

Dad often says that opportunity knocks only once , so one must take it when it comes.

爸爸总说机不可失、时不再来,有了机会一定要把握住.

互联网