Is "all but one" singular or plural? [closed]

Do you say “All but one person forgets something” or “All but one person forget something”?

I'm assuming that if all means five people, for example, then the example can be rewritten as Four people forget something, so the latter sentence would be correct, but I'm not sure.

Solution 1:

Plural. Don't get confused by the word "one" popping up in there. "One" is not the subject of the sentence. The entire phrase "all but one" is the subject, and it is treated as plural because it IS plural. It's a group.

Solution 2:

Since all swans but one are white is indisputably correct, I would perhaps say ?all but one swan are white; the singular *all but one swan is white doesn't make sense.

However, I think all swans but one are white is (nearly?) always superior to all but one swan are/is white, and so I would probably never write the latter either way. (But I would use all but one swan when it is not followed by a finite verb.)