When to use "once-in-a-lifetime" and when to use "once in a lifetime"?

Question

The first one has - connected and the others do not have, this two seem to have the same meaning but my teacher say not, what is the difference between them?

Answer 1

In general, hyphens are used to construct compound adjectives:

• I had to catch an early-morning train.
• I like late-night television.
• She wears extra-large socks.

Hyphens are not used when the words stand alone. So, to use your examples:

• It was a once-in-a-lifetime trip (a trip so special or expensive that one would be unlikely to undertake such a trip more than once, and once-in-a-lifetime acts as an adjective).
• We are born once in a lifetime (literally, this event happens exactly once, and the four words stand alone to describe that fact).