N3 · Probability and hearsay
Must be: にちがいない and にきまっている
strong conviction
Two ways to say that something must be so: one considered, one emphatic.
に違いない follows the plain form of a verb or i-adjective and comes directly after a noun or a na-adjective, without だ: 来るに違いない, 犯人に違いない. The polite form is に違いありません. Literally it means "there is no mistake that it is so".
It states a firm conviction that the speaker has reached from evidence: 電気がついている。家にいるに違いない, "the light is on — he must be at home". It is stronger than でしょう and はず, and it has a slightly written, considered tone; in novels and in reasoning aloud it is very common, in small talk less so.
に決まっている, literally "it is decided that", joins the same way and means "obviously, of course". It is spoken, emphatic and often emotional, and in casual speech it becomes に決まってる. Unlike に違いない it does not need evidence: people use it for what they are simply sure of, which is why it can sound like a prejudice.
English "must" also means "have to", and that meaning belongs to a different pattern entirely: 行かなければならない, "I must go". 行くに違いない means "he must be going — I am sure of it". Mixing the two is the classic error here.
The form
| verb, plain | 来るに違いない |
| verb, plain past | 来たに違いない |
| i-adjective | 高いに違いない |
| na-adjective (no だ) | 有名に違いない |
| noun (no だ) | 犯人に違いない |
| polite | 犯人に違いありません |
| emphatic, spoken | 嘘に決まっている |
| casual | 嘘に決まってる |
The rules one by one
Nouns and na-adjectives take it directly, without だ.
この足跡は猫のものに違いない。These footprints must be a cat's.
にちがいない is a firm conclusion drawn from evidence.
部屋の電気がついている。彼は家にいるに違いない。The light in his room is on. He must be at home.
The polite form is にちがいありません.
これは誰かの忘れ物に違いありません。Someone must have left this behind.
にきまっている is spoken and emphatic: obviously, of course.
そんな話、嘘に決まっているよ。A story like that is obviously a lie.
Neither means "have to"; for obligation use なければならない.
明日までにレポートを出さなければならない。I must hand in the report by tomorrow.
A closer look
How sure is sure
Lined up by confidence, the patterns of this area run from かもしれない through でしょう and はず to に違いない. The last two are close, but they rest on different things. はず works from known facts towards an expectation — the timetable says nine, so the train should arrive at nine. に違いない works from evidence towards something hidden — the footprints are small, so it must have been a cat.
に決まっている stands outside the scale. It expresses not how strong the evidence is but how strongly the speaker feels, and it can be wrong with great confidence. That is also why it is a little risky to use about other people.
How to spot it
- に違いない
- に違いありません
- に決まっている
- に決まってる
Example sentences
彼の顔を見れば分かる。何かあったに違いない。You can tell from his face. Something must have happened.
こんなにおいしいのだから、高いに違いない。It is so delicious that it must be expensive.
毎日練習しているのだから、上手になるに違いない。She practises every day, so she is bound to improve.
一人で夜の山に行くなんて、危ないに決まっているでしょう。Going up a mountain alone at night is obviously dangerous.
子どもはアイスクリームが好きに決まっている。Children love ice cream, of course.
犯人はこの中にいるに違いない。The culprit must be one of the people here.
7 more example sentences
あの店にはいつも行列ができている。おいしいに違いない。There is always a queue at that shop. It must be good.
彼女は今頃、空港に着いているに違いない。She must have reached the airport by now.
窓が割れている。誰かが入ったに違いない。The window is broken. Someone must have got in.
この字は父が書いたものに違いありません。This handwriting must be my father's.
試合はうちのチームが勝つに決まっている。Our team is going to win, obviously.
そんなに遅くまで起きていたら、朝起きられないに決まってるよ。If you stay up that late, of course you will not be able to get up in the morning.
あの二人は兄弟に違いない。顔がそっくりだ。Those two must be brothers. They look exactly alike.
Worth knowing (1)
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にきまっている can sound unfair
Because it needs no evidence, に決まっている is the natural way to express a strong opinion — and also a prejudice. あの人が悪いに決まっている, "it is obviously his fault", says more about the speaker than about the facts. With people you do not know well, と思います is the safer choice.
本当のことを言ったほうがいいに決まっています。It is obviously better to tell the truth.
The trap English speakers fall into
Watch out
The intended meaning is an obligation: "I must hand in the homework by tomorrow". に違いない is the "must" of certainty, not of duty, so the wrong sentence says "I am sure that the homework will be handed in". Obligation is なければならない.
にちがいない is must as in surely, never must as in have to.
Your turn
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