This phrase means that if something else were true, it would be okay. But because of the current situation, it is not okay or it is surprising.
Noun + なら + いざ知らずPrimary
Clause + の + なら + いざ知らず
This phrase means that if something else were true, it would be okay. But because of the current situation, it is not okay or it is surprising.
新入社員ならいざ知らず、入社10年目のベテランがこんな初歩的なミスをするとは信じられない。
If it were a new employee, that would be one thing, but for a 10-year veteran to make such a basic mistake is unbelievable.
子供のいたずらならいざ知らず、大人がやったにしては悪質すぎた。
If it had been a child's prank, that would be one thing, but for an adult to have done it, it was far too malicious.
１、２泊の旅行ならいざ知らず、一週間も家を空けるのに、ペットの世話を誰にも頼まなかったのですか。
A one or two-night trip would be one thing, but you were away for a whole week and didn't ask anyone to look after your pet?
一度聞いたぐらいで忘れてしまうのならいざ知らず、何度も説明したのに覚えていないのは問題だ。
If you forgot after hearing it just once, that would be understandable, but it's a problem that you don't remember even though I've explained it many times.
たまに食べるならいざ知らず、毎日そんなに油っこいものばかり食べていたら、体を壊しますよ。
If you ate it occasionally, that would be one thing, but if you eat such oily food every day, you'll ruin your health.
This phrase makes your sentence sound formal. It can show you are critical or complaining. It sets up a contrast. You are comparing one thing to another.
This is like 'if it were X, that would be okay, but...' It is stronger than '〜ならまだしも'. It sounds more formal. It also sounds a bit old-fashioned. '〜ならまだしも' is used more often in daily talk.
Don't use this when you want to end a sentence. For example, you cannot say "それはならいざ知らず" (sore wa nara izashirazu). This phrase usually starts a sentence.
Often confused with(2)
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