時不再來

Time that has passed will never come back. (idiom)

o'clock / time / when / hour / season / period

no; not so / (bound form) not; un-

no more / no longer

again; once more; re- / second; another / then (after sth, and not until then) / no matter how ... (followed by an adjective or verb, and then (usually) 也[ye3] or 都[dou1] for emphasis)

to come / to arrive / to come round / ever since / next

Opportunity knocks but once. (idiom)