grammar - Can an adverb be a predicate?

Consider the following quote,

“Life is once. Forever.” —Henri Cartier-Bresson, AZ Quotes

I have checked that "once" is an adverb. How can it become a predicate?

Answer

Adverbs could be part of a predicate, to be more exact, a subjective complement, but it doesn't happen very often and they are used either idiomatically or in a sentence where a participle or part of the sentence is elided.

In your quote, "Life is once" could be considered as short for

Life is given to people once. Life is lived once by
people.

and it doesn't necessarily mean once is a subjective complement alone once elided words are placed. The past participle given or lived is elided in your example.

He will be (coming) back in 5 minutes: coming is elided.

He is out (of the house/building/restaurant...): a place is elided.

The secret is out: the secret has been discovered/revealed/known.

There are more examples where an adverb could be used as a subjective complement. They should be learned on a case-by-case basis.