What's the difference between 「いけない」 and 「いかない」?

Question

I've just learned that `～わけにはいけない` as in `断{ことわ}るわけにはいけない` ("I can't refuse") apparently isn't valid but `～わけにはいかない` is, even though I've been using the former frequently. But what is the difference between `いかない` and `いけない` and when should they each be used? Also, is `(te form)はいかない` valid? I'm guessing `～してはいかない` isn't valid but `～してはいけない` is. Is `うまくいけない` or `納得いけない` valid? In English I think "this can't go well" or "I can't tolerate that" sounds like it would work, but I'm not sure it would in Japanese.

Answer 1

いけない refers to the "potential" form of the verb, and would translate roughly as "be able to" This is why it is used in the"TE form" permission-prohibition structure, for instance たばこをすってはいけません。(Literally 'you are not able to smoke here).

いかない　is the short form negative of the verb いく。Translated, it means "not go."