N4 · Using the te-form
てもいい and てはいけない: may, must not, need not, must
permission and obligation
Four patterns built on te-forms that allow, forbid, excuse and oblige.
Permission is the te-form plus もいい: ここに座ってもいいですか, "may I sit here?" The も means "even": literally "even if I sit here, is it all right?" The answer to a request like this is はい、どうぞ or いいですよ.
Prohibition is the te-form plus は and いけない, polite いけません: ここで泳いではいけません, "you must not swim here" — literally "if you swim here, it will not do". It is strong: the language of rules, notices and parents. In casual speech だめ often takes the place of いけない.
The other two are built on the ない-form. "Need not" is なくてもいい: 来なくてもいいです, "you don't have to come". "Must" is なければならない or なくてはいけない: 明日は早く起きなければなりません, "I have to get up early tomorrow". Literally: "if I don't, it will not do" — a double negative, which is why it is so long.
The pair English speakers mix up is "must not" and "don't have to". In English they look alike and mean opposite things; Japanese keeps them apart, and confusing てはいけない with なくてもいい forbids what you meant to excuse. In speech you will also hear the short forms ていい, ちゃだめ, なきゃ and なくちゃ; recognising them matters more than producing them.
The form
stem (regular) stem (irregular) ending
| may eat | 食べてもいい |
| may write | 書いてもいい |
| must not eat | 食べてはいけない |
| must not swim | 泳いではいけない |
| need not eat | 食べなくてもいい |
| need not come | 来なくてもいい |
| must eat | 食べなければならない |
| must do | しなければならない |
| must go | 行かなくてはいけない |
| spoken: must go | 行かなきゃ |
| spoken: must not go | 行っちゃだめ |
The rules one by one
Te-form + もいい gives or asks for permission.
窓を開けてもいいですか。May I open the window?
Te-form + はいけない forbids.
美術館の中で写真を撮ってはいけません。You must not take photographs inside the museum.
The ない-form with くてもいい says something is not necessary.
土曜日は会社に来なくてもいいです。You don't have to come to the office on Saturday.
The ない-form with ければならない or くてはいけない says something must be done.
毎日薬を飲まなければなりません。I have to take medicine every day.
A request for permission is usually refused indirectly, not with いけません.
すみません、ここはちょっと…。I'm sorry, that's a bit difficult here…
A closer look
Why "must" is a double negative
なければならない takes apart as なければ, "if not", and ならない, "it will not do". Japanese states an obligation by saying that the alternative is impossible. The variants are built the same way: なければいけない sounds a little more personal, なければならない suits general rules and writing, and ないといけない is very common in conversation.
In casual speech the end is usually cut off: なければ becomes なきゃ, なくては becomes なくちゃ, and the rest is left to be understood. もう行かなきゃ is "I've got to go". The same happens to the prohibition: ては becomes ちゃ, and 触っちゃだめ is "don't touch".
Exceptions and special cases (1)
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what comes before もいい is an adjective or a noun
an i-adjective takes くても, a na-adjective or a noun takes でも: 高くてもいい, 明日でもいいです.
小さくてもいいですから、静かな部屋をお願いします。It can be small — I would like a quiet room, please.
The pattern is "even if X, it is fine", and X can be any te-form, not only a verb's.
How to spot it
- てもいい
- てもいいですか
- てはいけない
- ちゃだめ
- なくてもいい
- なければならない
- なくてはいけない
- なきゃ
Example sentences
この部屋でたばこを吸ってもいいですか。May I smoke in this room?
試験で辞書を使ってはいけません。You may not use a dictionary in the exam.
明日は休みだから、早く起きなくてもいいです。Tomorrow is a day off, so I don't have to get up early.
レポートは金曜日までに出さなければなりません。The report has to be handed in by Friday.
全部食べなくてもいいですよ。You don't have to eat it all.
ここに車を止めてはいけません。You must not park here.
8 more example sentences
鉛筆で書いてもいいですか。May I write in pencil?
子どもは夜一人で出かけてはいけません。Children must not go out alone at night.
予約しなくてもいいですか。Is it all right if I don't book?
もう帰らなければなりません。I have to go home now.
週末も働かなくてはいけません。I have to work at the weekend too.
このペン、借りてもいい？Can I borrow this pen?
明日は試験だから、勉強しなきゃ。I've got an exam tomorrow, so I have to study.
会議は三時からでもいいですか。Would it be all right if the meeting started at three?
Worth knowing (1)
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Asking permission more carefully
In polite company the request is softened further: てもよろしいですか replaces てもいいですか, and てもいいでしょうか sounds less sure of the answer. A refusal is rarely said outright with いけません, which sounds like a rule being read aloud; people trail off with ちょっと… and let the listener understand.
こちらに座ってもよろしいですか。Would it be all right if I sat here?
The trap English speakers fall into
Watch out
If you mean "you don't have to come tomorrow", the form is なくてもいい. 来てはいけません is "you must not come" — a ban, not a release. English "must not" and "don't have to" sound close; in Japanese they do not.
てもいい permits, てはいけない forbids, なくてもいい excuses, なければならない obliges.
Your turn
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