〜てもいい permits; 〜てはいけない prohibits; 〜なければならない requires; 〜なくてもいい says something is unnecessary. 行ってもいい means “you may go,” and 行かなくてもいい means “you need not go.” 行かなければならない means “you must go,” while 行ってはいけない means “you must not go.” Casual speech often shortens obligation forms to 行かなきゃ or 行かないと.
also written temoii / nakereba naranai / てはいけない / なくてもいい / なきゃ / ないと
てもいい — you may · てはいけない — you must not · なければならない — you must; なくてもいい — you need not
Sentences
6 sentences, word by word
てもいいですか — asking permission
polite
？
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写真
しゃしん ［0］
photograph
noun · 写真を撮る is the set phrase
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を
を
marks the direct object
particle · pronounced お
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撮って
とって
take
て-form of 撮る · 撮る for photos, not 取る
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も
も
too; as well
particle · replaces を and が, never stacks with them
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いい
いい
is fine
い-adjective · 'even if you do, it's good'
てもいいN4may · permission
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です
です
(polite ending)
copula · politeness on an い-adjective predicate
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か
か
(question)
particle · which is what makes it a request rather than an offer
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May I take a photo?
てはいけない — prohibition
polite
。
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ここ
ここ
here
pronoun · the place near the speaker
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で
で
in; at — where it happens
particle · where the action happens, or the manner it happens in
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写真
しゃしん ［0］
photograph
noun · the same set phrase, now forbidden
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を
を
marks the direct object
particle · pronounced お
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撮って
とって
take
て-form of 撮る · joining to はいけません
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は
は
as for — the topic
particle · written は, read わ
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いけません
いけません
won't do; is not allowed
polite negative potential of いく · the prohibition half of the grid
てはいけないN4must not
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You mustn't take photos here.
なければなりません — obligation
polite
。
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明日
あした ［3］
tomorrow
noun used adverbially · the deadline
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までに
までに
by (a deadline)
particle · a point by which it must be done
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宿題
しゅくだい ［0］
homework
noun · 宿題を出す, to hand it in
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を
を
marks the direct object
particle · pronounced お
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出さなければ
ださなければ
if I don't hand it in
ない-form of 出す minus い, plus ければ · the first negative
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なりません
なりません
it won't do
polite negative of なる · the second negative, giving obligation
なければならないN4must
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I have to hand in my homework by tomorrow.
なくてもいい — you don't have to
polite
。
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靴
くつ ［2］
shoes
noun · 靴を脱ぐ is the set phrase
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を
を
marks the direct object
particle · pronounced お
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脱がなくて
ぬがなくて
not taking off
ない-form of 脱ぐ minus い, plus くて · the negative て-form
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も
も
too; as well
particle · replaces を and が, never stacks with them
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いい
いい
is fine
い-adjective · 'even not doing it is fine' — release, not prohibition
なくてもいいN4need not
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です
です
(polite ending)
copula · politeness on the い-adjective
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You don't have to take your shoes off.
なきゃ — the spoken contraction
plain
。
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もう
もう
already
adverb · marking that the time has come
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帰らなきゃ
かえらなきゃ
gotta go home
contraction of 帰らなければ · the ending is simply dropped
なきゃN4gotta · spoken
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I've got to go home already.
ないと — the other spoken option
plain
、。
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急がないと
いそがないと
if we don't hurry
ない-form of 急ぐ ＋ と · the consequence is left unsaid
ないとN4have to · spoken
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遅れる
おくれる ［0］
we'll be late
ichidan verb · the consequence, here actually stated
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よ
よ
you know
sentence particle · pressing the point
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We have to hurry or we'll be late.
Structure
How the four are built
Verb ［て］＋もいい
帰ってもいい. As a question — 帰ってもいいですか — it is the standard request for permission.
Verb ［て］＋はいけない
入ってはいけない. Prohibition. Contracts to 入っちゃだめ in speech, which is softer and much more common between friends.
Verb ［ない minus い］＋ければならない
行かなければならない. Built on the ない-form, not the て-form — 行かな＋ければ. This is where the shape trips people.
Verb ［ない minus い］＋くてもいい
行かなくてもいい. You don't have to go. Same ない-form base, joined with くても instead.
Meaning
ならない vs いけない vs だめ
The grid.てもいい permits, てはいけない prohibits, なければならない requires, なくてもいい releases. Note that the opposite of “you must” is “you need not,” not “you must not” — the pattern learners most often reach for wrongly.
The obligation forms are double negatives.行かなければならない is literally “if you do not go, it will not do.” Nothing in it means “must”; the obligation falls out of negating a negative, which is why the shape looks so unlike its meaning.
ならない and いけない lean different ways.ならない tends toward general rules and written Japanese; いけない toward this particular situation and speech. It is a tendency rather than a rule, and in most sentences either is accepted.
Speech contracts all of it.なければ → なきゃ, なくては → なくちゃ, and the ending usually drops off entirely: 行かなきゃ is a complete sentence. Keep the full form for writing.
〜ないと is the third spoken option.行かないと — the conditional と with the consequence left unsaid. Very common, entirely casual, and the same trick as なきゃ: the threat is implied.
Watch out
Where the four go wrong
Treating てはいけない as the opposite of なければならない
The opposite of “you must” is “you need not” — なくてもいい. てはいけない is the opposite of てもいい. Two axes, not one.
Building the obligation form on the て-form
行ってなければならない is not a thing. The base is the ない-form with its final い removed: 行かな＋ければ.
Writing なきゃ or ないと
Both are speech. In an email, an essay, or anything read by a superior, use なければなりません — and note that なきゃ normally drops its ending entirely, which is part of why it reads as casual.
Also asked
Questions people ask about these four
What's the difference between ならない and いけない?
A tendency rather than a rule. ならない leans toward general rules and writing; いけない toward the situation in front of you and speech. In most sentences either is accepted.
Why is “must” a double negative?
行かなければならない is literally “if I don't go, it won't do.” The obligation is the product of two negatives, which is why nothing in the shape looks like “must.”
What's the opposite of なければならない?
なくてもいい, “you don't have to.” Not てはいけない — that is the opposite of てもいい.
How do I ask permission politely?
〜てもいいですか is the everyday form. To go further up, 〜てもよろしいでしょうか, or the humble 〜させていただけますか.
Is なきゃ rude?
Casual rather than rude. It is fine with friends and family and out of place in writing or with a superior, where なければなりません belongs.
What is 〜ちゃだめ?
The spoken contraction of 〜てはだめ, itself a softer 〜てはいけない. 食べちゃだめ — don't eat that. Common to children and between friends.
