ikenai!

[jampanman]

Hi,

I'd like to know what the difference in usage is between ~てはいけない and ~なければいけない.

Thanks ahead.

[QuackingShoe]

してはいけない Literally a continuation of する to いけない, which means 'can't go,' with cultural connotations of 'bad'. You can't/shouldn't do it, it's a bad idea, even your mother will hate you if you try.
しなければいけない. Literally if you don't do, (whatever) can't go. You have to/should do it.

In other words, they have completely opposite meanings.

[hatch_jp]

~てはいけない
= mustn't ～ / be not allowed to ～
食べてはいけない ＝ You mustn't eat it. / You are not allowed to eat it.

~なければいけない.
= have to ～ / must ～
食べなければいけない = You [have to / must ] eat it.

[Javizy]

いけない can be used by itself to mean 'wrong, not good', like 「老人をだますなんていけない」 'conning elderly people is wrong', that's why you can replace it with だめ in these two constructions.

「食べちゃだめ！」 'you can't eat it!'
「食べなきゃだめ！」 'you have to eat it!'

It makes understanding these expressions less confusing than trying to relate them to the verb 行く.

[Taurus]

Hi,

I'd like to know what the difference in usage is between ~てはいけない and ~なければいけない.

Thanks ahead.

I think you might be confusing ではいけない with ないといけません, which, I think I'm correct in saying, means pretty much the same as なければいけない. I'm not sure if the two have a different nuance or anything...