At a glance
Xならいざ知らず means "X might be one thing, but..." or "I could understand it if it were X, but...". The part before it is a case where something would be acceptable. The main clause describes the real situation, where it is not acceptable.
[Noun] + ならいざ知らず、[main clause]
[Plain-form verb / adjective] + ならいざ知らず、[main clause]
子供ならいざ知らず、大人がこんなミスをするなんて。 A child, maybe, but for an adult to make a mistake like this!
How to form it
It most often follows a noun, but plain-form verbs and adjectives can also come before it.
| Attaches to | Form | Example |
|---|---|---|
| Noun | Noun + ならいざ知らず | 子供ならいざ知らず |
| Verb | Plain form + ならいざ知らず | 知らなかったならいざ知らず |
| い-adjective | Plain form + ならいざ知らず | 近いならいざ知らず |
| な-adjective | Stem + ならいざ知らず | 暇ならいざ知らず |
Register: formal and rather stiff. It is mostly found in writing and formal speech.