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

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.

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They should be from different verbs.　行ける > いけない and 行く > いかない. But I'm not familiar with 行ける at all so I can't say for sure what the difference is. –  Flaw Nov 21 '11 at 2:21
Alternatively they can be from the same verb. 行く +(short potential form) to get 行ける and then +negative to get いけない. This would mean "Cannot go". Compare with 行かない which means "will/does not go". –  Flaw Nov 21 '11 at 2:33
weird, were you looking at my post on another site? i got a bunch of answers about this on lang8 lang-8.com/315974/journals/1188521/… tell me if that link works –  yadokari Nov 21 '11 at 4:30
@yadokari yeah, that's where I realized my mistake. Hope that was OK :P I could tell that what I was doing was wrong by the answers on that page, but I'd like to know more about why it's wrong as I didn't fully understand. –  cypher Nov 21 '11 at 4:51
thats cool. a way i think of it is this それはいけない can mean "that's bad" (its often used that way) you can equate it to the american slang "don't go there" which often has nothing to do with the literal meaning but in the same way means "that's bad". a closer translation would be, we can't go there- we can't breach that topic we cant do that thing we cant cause such an infraction = thats bad それはいかない：just means i don't go but can also mean I don't want to go…行きたくないという強い意志があります。 i think its used less than the other in the figurative sense –  yadokari Nov 21 '11 at 5:06

いかない is less often a problem as it's derived from 行く, but いけない can be both the negative potential form of the verb 行く "to go" and the negative form of the verb 行ける "to be good (at)/to go well/to be acceptable." 6

Therefore いかない and いけない may appear to be similar, but they can actually be totally different.

いかない "won't go/doesn't go" from 行く:

～わけにはいかない: An expression which means "I cannot very well" when after a verb that isn't in the past tense. 5

• いつまでもけんかしているわけにはいかない "I can't very well keep fighting forever." 7

いけない "unable to go/can't go" from 行く:

そのパーティーにいけない: Can mean　"I can't go to the party." 2

いけない "unacceptable/bad" from 行ける 5:

～てはいけない: Can mean "unacceptable if" however it's more often translated as "must not/may not." Means "must/have to" if on the end of a negative verb. 5

• 走{はし}ってはいけない "you must not run."
• 走らなくてはいけない "you must run."

～はいけない: Can mean "won't do/no good/bad" as well as "naughty."

• 彼{かれ}はいけない "he is naughty." 1
• それはいけない "that's bad" in many contexts. 2

～て（で）いけない: Means "undesirable/unpleasant" after a conjunctive form.

• いい人だが、おしゃべりでいけない "A great person, but his/her chattering isn't to be desired." 4

～といけないから… or ～といけないので… etc: On the end of verbs means "it will be bad if ~ happens so ..."

• 雨{あめ}が降{ふ}るといけないから傘{かさ}を持{も}ちなさい "It will be bad if it rains so take an umbrella." 1

• お前がよく[注意]{ちゅうい}していなかったのがいけないんだ "It was your fault that you weren't careful." 3

"Hopeless": (駄目{だめ}) if used as もういけない or やっぱりいけなかった.

• あの病人{びょうにん}はもういけないようです "It seems that patient is beyond hope." 1
• 試験{しけん}はやっぱりいけなかった "Just as I feared, I've failed the exam." 1

• それはいけません in response to hearing that someone is unwell would mean "it's unfortunate that you are unwell." 1

Other meanings:

• "Beyond hope of recovery" if used as いけなくなった 3
• "Unable to drink alcohol" as in あまりいけない口でして 4
• "Darn!" when by itself, similar to しまった or ちくしょう 1
• Used by females as in あ、いけない、どうしよう "Oh dear, now what am I going to do?" 1

Sources:

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Is this not getting upvoted because something is lacking? –  Louis Nov 30 '11 at 2:04
@Louis I'm not sure, but I'll try to clean up the formatting and go through all the sources again to see if there's anything I've missed etc. –  cypher Dec 2 '11 at 6:45