Is there any difference between わけにはいかない、～てはいけない and ～てはならない？


From what I understand, they all mean the same, but maybe the nuance is
different or it could be something else.




There is no semantic difference between ～てはいけない and ～てはならない. They both simply
mean "must not", but the latter is stiffer and more literary. The latter is
naturally used in formal essays, legal documents, etc. In speech, ～てはいけない is
the normal choice unless you are imitating a samurai or speaking with a
serious atmosphere.

～わけにはいかない is never used to express simple "Don't" or "must not". わけ means
"reason", so ～わけにはいかない is more nuanced. There is a situation-dependent
"reasoning" when someone says ～わけにはいかない. It means something along the lines of
"～ is not going to make sense", "～ won't do in this situation", "had better
not do ～ (otherwise ...)", or "～ is not a reasonable choice (now)".




There is no semantic difference between ～てはいけない and ～てはならない. They both simply
mean "must not", but the latter is stiffer and more literary. The latter is
naturally used in formal essays, legal documents, etc. In speech, ～てはいけない is
the normal choice unless you are imitating a samurai or speaking with a
serious atmosphere.

～わけにはいかない is never used to express simple "Don't" or "must not". わけ means
"reason", so ～わけにはいかない is more nuanced. There is a situation-dependent
"reasoning" when someone says ～わけにはいかない. It means something along the lines of
"～ is not going to make sense", "～ won't do in this situation", "had better
not do ～ (otherwise ...)", or "～ is not a reasonable choice (now)".

