Japatan
Formation
- N + ならいざしらず
- V普通形 + のならいざしらず
- N + はいざしらず
Explanation
A ならいざしらず B means 'it might be understandable in the case of A, but B is different.' A is a special or extreme case (a child, a beginner, the old days), and B expresses the speaker's surprise, criticism or firm judgment, such as 'that is strange' or 'that can't be.' いざしらず comes from an old expression meaning 'I don't know about that, but...'.
Examples
子どもならいざしらず、大人がこんな簡単なルールを守れないのは困る。
It would be one thing if it were a child, but it's a problem when an adult can't follow such a simple rule.
初心者ならいざしらず、十年も働いている人がこのミスをするとは思わなかった。
I could understand it from a beginner, but I never thought someone with ten years of experience would make this mistake.
知らなかったのならいざしらず、知っていて黙っていたのなら責任がある。
If you hadn't known, that would be different, but if you knew and kept quiet, you are responsible.
昔ならいざしらず、今はスマートフォンで何でもすぐに調べられる。
It may have been different in the past, but now you can look up anything instantly on a smartphone.
Notes
The second half usually carries a strong judgment or criticism. It is not used simply to add something ('not only A but also B'); use はもちろん for that. It is a formal expression.
Related grammar
Mini quiz
1旅行者（ ）、地元の人がこの道を知らないはずがない。
Answer and explanation
Answer: 3 ならいざしらず
"A tourist might not know, but there's no way a local doesn't know this road." ならいざしらず contrasts an excusable case with this one. The other forms do not connect the two halves logically.
2「ならいざしらず」の使い方がいちばん正しい文はどれですか。
Answer and explanation
Answer: 1 日曜日ならいざしらず、平日の朝にこんなに混むのはおかしい。
1 means "It would make sense on a Sunday, but it's strange that it's this crowded on a weekday morning." 2 and 4 have no contrasting judgment, and 3 needs はもちろん (not only adults but also children).
3一年生ならいざしらず、四年生が図書館の場所を（ ）。
Answer and explanation
Answer: 4 知らないのはおかしい
"A first-year student maybe, but it's strange that a fourth-year student doesn't know where the library is." 1 and 3 would describe the excusable case, and 2 reverses the meaning.
