ならいざしらず and はいざしらず set one case aside – "X might be another matter, but…" – and then say that the case at hand is different, usually worse. Both follow a noun; ならいざしらず also follows a な-adjective directly, or a plain-form verb or い-adjective.
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I could understand it for a child, but it's embarrassing for an adult to do something like that.
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I don't know about the past, but that kind of thinking doesn't fly in this day and age.
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One or two mistakes might be forgivable, but having ten is a problem.
is literally "well, I couldn't say", and it is written in kanji as
just as often. The first case is usually the extreme or unlikely one – a child, the distant past, an expert, a tiny number – and the second half criticises, complains or says something can't be done.
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A day or two, maybe, but I can't wait a whole month.
ならいざしらず or はいざしらず
With なら, the first case is hypothetical: "if it were X". With は, it is simply a topic set aside: "as for X, I can't say". The meaning barely changes.
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It might be possible for a professional, but this job is impossible for an amateur.
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I don't know about professionals, but this job is impossible for an amateur.
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I don't know about anyone else, but I'm against it.
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Both forms are formal and mostly written: essays, speeches, editorials. In conversation, does the same job, and is close to it.
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For a child, sure, but it's embarrassing for an adult to do that.
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A day or two would be fine, but I can't wait a whole month.
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