Grammar detail: ないわけにはいかない

JLPT:

ないわけにはいかない - can't very well not do; have to do

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When わけにはいかない follows a negative clause, as we would expect 'can't very well ~' now becomes 'can't very well *not* ~'. In English a more natural way of saying 'can't very well not' would often be 'have to do' or 'must do'.

あんなに面白そうな映画を、見に行かないわけにはいかない。

I can't very well not go and see such an interesting-sounding movie.

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もやらないわけにはいかないね。
You can't just do nothing.

'Have to do' or 'must do' are also how two other expressions 'しなければなりません' and 'しなくてはいけません' might be translated. However, where those expressions imply a legal or formal requirement to do something, わけにはいかない implies more of a sense of 'moral' obligation. For example, if a worker made an error, the manager may well say:

部下を怒らないわけにはいかない

I can't very well not get angry. I can't very well let this go.

However, the manager would be unlikely to use one of the other expressions, such as 部下を怒らなければならない, because that would mean 'I must get angry (according to the rule book).'

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