must not・may not
Structure
- Verb[て] + は・いけない
meaning
must not・may not
This is another grammar point where you should probably also mention the 漢字 spelling:
ては行けない
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Generally, when the verb is used as auxiliary etc, it is written using kana alone. That way it is distinguished from using it as a standalone verb.
That’s why てはいけない、ておく(て置く)、てみる(て見る) are all written in kana. Using kanji may confuse readers.
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This says to use long form て, but in 車で いってはいけない, いって is the shortest て form the verb has, isn’t it? Am I missing something? I thought the long form was 行かなくて, as when you’re saying you must do something.
Thanks,
Cris
@CrisH Hey! I think that the confusion may have come from the wording of the hint, with “て + いく” immediately following “long form.” The long form refers to the whole combination of て + は + いけない as opposed to shortened phrases like いっちゃだめ, or いったアカン (Kansai dialect). Cheers!
Ah, that’s probably it. I haven’t come across that yet, so I assumed long form was 行かなくて and short form was 行って. Thanks for clarifying 
@CrisH 行かなくて and 行かないで are the negative て-forms of the verb 行く, while 行って is the plain て-form. 行かなくてはいけない means “(you) must go,” while 行ってはいけない means “(you) must not go.” Cheers!
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Does anyone know why は is used in this construction and how it interacts with a verbs て-form? I am just curious is all. 
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What’s up with the sudden “いけません” in the “この池で泳いではいけません 。” example? It’s not mentioned in the Meaning section.
Hey and long time no see 
てはいけません is a polite form of てはいけない. It is made just like the ordinary polite negative form of a verb.
で comes from the fact that the てform of 泳ぐ is 泳いで (it is still called a て form even though technically for those verbs its で).
So:
泳いで - て form of verb 泳ぐ
泳いでいけない - short form
泳いでいけません - polite form
I hope it helps,
Cheers
Ask if you have any questions, I have caught a small cold so I might not explain clearly 
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If anyone was wondering where in [DBJG] this grammar is, its on page 528.
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Why does the hint say て＋いくinstead of て＋いける? This confused me a bit.
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@lasyat Hey! The hints are just there to steer you clear of using something like てはならない. If we provide too much of the actual grammar point (the partial conjugation in this case) then it becomes too easy to answer. The hint is just meant to get you in the right mindset (okay, this question is asking me to conjugate to the long form of て (as opposed to なくちゃ or なきゃ) first and then not use ならない). Cheers!
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Hi,
I would suggest to include “は + いけません” in the meaning box as well.
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Not that it really matters, but it looks like there’s a rogue space in the title of this topic
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Included 
Fixed! It might be useful especially for those, who use search feature on forums.
Cheers!
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If this conjugation uses the verb 行く, why does the polite negative and non-polite negative conjugate as いけない and いけません instead of いかない and いきません like 行く does normally?
This construction uses the potential form of 行く (行ける, “can go”), not the dictionary form.
So when conjugating it into the negative we get いけない/いけません (行ける itself is an ichidan verb).
The literal translation for Aてはいけない is then something like “doing A cannot go”, which is better interpreted as “must not do A”.
Ah, that makes a a lot of sense, thank you!