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

Wednesday, 22 July 2015

grammar - 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.

Answer

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

"It's your fault that":

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

"Unfortunate/too bad" (expressing sympathy):

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