Explanation
Expresses 'maybe X is possible, but Y is definitely not'. Often 'let alone...'.
Formation
Noun1 + ならいざ知らず + Noun2 (comment)
Detailed explanation
「Xならいざ知らず、Yは…」 emphasizes that while X might be possible or acceptable, Y is out of the question or even more difficult. It can be translated as 'I could understand X, but Y is impossible' or 'Maybe X, but definitely not Y.'
Examples
Suugaku nara iza shirazu, butsuri nante zenzen wakaranai.
Math might be manageable, but as for physics, I'm completely clueless.
Go kiro nara iza shirazu, juu kiro nante hashiremasen.
Maybe I could run 5 kilometers, but 10 kilometers is out of the question.
Nihongo nara iza shirazu, chuugokugo nante hanasenai.
I might be able to speak Japanese, but Chinese is impossible for me.
Tsumetai mizu nara iza shirazu, atsui mizu de oyogu nante kangaeta koto mo nai.
Maybe cold water, but swimming in hot water never even crossed my mind.
Practice
Grammar content sourced from hanabira.org.