でなくてなんだろう follows a noun and states, as strongly as the language
allows, that something is that noun: "if this isn't X, what is?" The question
is rhetorical, and the only possible answer is X. The thing being judged
usually takes , or for extra weight.
。
If this isn't love, what is?
。
If his actions aren't betrayal, what are they?
。
If risking your life to save someone isn't courage, what is?
に違いない or でなくてなんだろう
is a guess from evidence: "must be". でなくてなんだろう is not a
guess. The speaker already knows the facts and names them – love, fate, a
miracle – with as much force as possible, so it takes a noun for a quality or
a judgement, never a verb.
、。
The light is on, so Tanaka must be at home.
。。
She waited ten years for him. If that isn't love, what is?
- × 田中さんは家にいるでなくてなんだろう → 田中さんは家にいるに違いない
Usage notes
These go beyond the plain だろう ending.
だろうか
With か on the end the question is spelled out in full. The meaning does not
change.
。
If this isn't a miracle, what is?
であろう
in place of だろう is the most literary form, found in essays and
speeches.
。
If this is not art, what is?
Register
The pattern belongs to writing: essays, speeches, a novel's narration at an
emotional peak. Said across a table it sounds theatrical. In conversation,
or carries the same conviction.
。
It's nothing short of a miracle.
Lingaku