modality N1 rare literarywritten
〜でなくてなんだろう — if this isn't ~, then what is it?
〜でなくてなんだろう ・ でなくてなんだろう
Builds on だろう
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See 〜でなくてなんだろう in real sentences
Jengo shows 〜でなくてなんだろう the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled from published Japanese grammar references. Editorial confidence: medium.