Meaning & Usage
The N2 grammar pattern 「に違いない」 (ni chigainai) expresses a speaker's strong conviction or certainty about a situation. It translates to phrases like "must be," "no doubt," "I'm sure that," or "it's certain that." This expression is used when you have observed specific evidence, considered logical reasoning, or possess information that leads you to a firm, almost inescapable conclusion. It signifies a high degree of probability, bordering on certainty, always based on inference rather than direct knowledge.
Think of 「に違いない」 as the Japanese equivalent of a logical deduction. When you use it, you're asserting that, given the circumstances, the conclusion is almost unavoidable. It conveys a stronger sense of conviction than merely saying something "is probably" the case (～でしょう / ～だろう) and implies a deeper level of analysis or confidence than just an expectation (～はずだ).
For example, imagine you see someone in a chef's uniform carrying a large bag of ingredients. You might then deduce, 「あの人は料理人に違いない」 (That person must be a chef). The visible evidence—the uniform and ingredients—leads to a strong, logical conclusion. It’s not simply a guess; it's an inference drawn from clear signs.
The nuance of 「に違いない」 leans towards formality, making it more common in written language, speeches, or serious discussions. While it can be used in spoken Japanese, especially when making a strong point, it might sound a bit assertive or academic in very casual conversation. In informal contexts, softer expressions like 「～だろう」 or even the simpler 「絶対(ぜったい)～だ」 might be preferred, though they don't carry the same weight of logical deduction. However, 「に違いない」 is particularly effective when you want to emphasize that your conclusion is well-founded and almost undeniable.
A helpful mental model for 「に違いない」 is a courtroom lawyer presenting a closing argument, or a detective summarizing their findings. They aren't just 'thinking' something is true; they are presenting evidence that leads to the firm conclusion that it 'must be' true. This phrase powerfully highlights the speaker's confidence in their own judgment, backed by available data.
Structure & Formation
The grammar pattern 「に違いない」 attaches to the plain form (also known as dictionary form or casual form) of verbs and い-adjectives. For な-adjectives and nouns, it connects using their stem followed by 「だ」.
It's crucial to connect 「に違いない」 correctly to ensure grammatical accuracy.
| Word Type | Connection Rule | Example | Romaji | Translation |
|---|---|---|---|---|
| Verb | Plain Form + に違いない | 彼は来るに違いない。 | Kare wa kuru ni chigainai. | He must come. |
| い-adjective | Plain Form + に違いない | これは美味しいに違いない。 | Kore wa oishii ni chigainai. | This must be delicious. |
| な-adjective | Stem + だ + に違いない | 彼女は元気だに違いない。 | Kanojo wa genki da ni chigainai. | She must be fine. |
| Noun | Noun + だ + に違いない | あれは本当だに違いない。 | Are wa hontou da ni chigainai. | That must be true. |
Important Note for な-Adjectives and Nouns: The standard plain form connection for these word types includes 「だ」 (e.g., 元気だ, 本当だ). In some contexts, particularly in speech, this 「だ」 can be omitted, resulting in forms like 「元気に違いない」 or 「本当に違いない」.
However, when writing or in more formal Japanese, or when emphasizing the declarative nature, including 「だ」 is generally considered more complete and grammatically standard for this pattern.
Example Sentences
General Deductions and Certainty
彼は昨日寝ていないから、今疲れているに違いない。
Kare wa kinou nete inai kara, ima tsukarete iru ni chigainai.
He didn't sleep yesterday, so he must be tired now.
この味は母の料理だ。間違いない、母が作ったに違いない。
Kono aji wa haha no ryouri da. Machigainai, haha ga tsukutta ni chigainai.
This taste is my mother's cooking. No doubt, my mother must have made it.
外が暗いし、もう夜中に違いない。
Soto ga kurai shi, mou yonaka ni chigainai.
It's dark outside, so it must be midnight already.
あの荷物の量からして、旅行に行くに違いない。
Ano nimotsu no ryou kara shite, ryokou ni iku ni chigainai.
Judging from the amount of luggage, they must be going on a trip.
Making Strong Assertions about the Past or Future
彼女はいつも真面目だから、試験に合格したに違いない。
Kanojo wa itsumo majime da kara, shiken ni goukaku shita ni chigainai.
She's always serious, so she must have passed the exam.
この古い建物は、昔は教会だったに違いない。
Kono furui tatemono wa, mukashi wa kyoukai datta ni chigainai.
This old building must have been a church in the past.
彼の表情からすると、何か良いことがあったに違いない。
Kare no hyoujou kara suru to, nani ka yoi koto ga atta ni chigainai.
Judging by his expression, something good must have happened.
この状況では、計画を変更するに違いない。
Kono joukyou de wa, keikaku o henkou suru ni chigainai.
In this situation, they must change the plan.
Observations and Inferences
窓が開いているから、誰かがここにいるに違いない。
Mado ga aite iru kara, dareka ga koko ni iru ni chigainai.
The window is open, so someone must be here.
この足跡は、熊の足跡に違いない。
Kono ashiato wa, kuma no ashiato ni chigainai.
These footprints must be bear footprints.
店の前に長い列ができている。有名なレストランに違いない。
Mise no mae ni nagai retsu ga dekite iru. Yuumei na resutoran ni chigainai.
There's a long line in front of the store. It must be a famous restaurant.
あの選手は毎日練習している。優勝するに違いない。
Ano senshu wa mainichi renshuu shite iru. Yuushou suru ni chigainai.
That athlete practices every day. They must win the championship.
彼がここまで熱心に話すのは、本当にこの計画を信じているに違いないからだ。
Kare ga koko made nesshin ni hanasu no wa, hontou ni kono keikaku o shinjite iru ni chigainai kara da.
The reason he speaks with such enthusiasm is that he must truly believe in this plan.
Common Mistakes
Mistake 1: Using for personal desires or vague possibilities
「に違いない」 expresses a high degree of certainty based on objective reasoning or evidence. It's not suitable for subjective wishes, vague possibilities, or things you simply hope are true.
❌ 明日は晴れるに違いない。（ただの希望）
✅ 天気予報によると、明日は晴れるに違いない。（根拠がある）
The first sentence sounds like a mere hope or wish for good weather. In contrast, the second one provides a clear basis for the strong assertion by including "according to the weather forecast," which makes 「に違いない」 appropriate. If there's no clear evidence, use expressions like 「～といい」 (I hope), 「～でしょう」 (probably), or 「～はずだ」 (should be).
Mistake 2: Confusing with direct knowledge or facts
「に違いない」 is for inferences or strong deductions. It's not used when you already have certain knowledge or are stating an undeniable fact.
❌ 富士山は日本で一番高い山だに違いない。
✅ 富士山は日本で一番高い山だ。
The height of Mt. Fuji is a known fact, not something to be strongly inferred. Simply stating the fact is sufficient. 「に違いない」 implies the speaker arrived at the conclusion through reasoning, not prior knowledge.
Mistake 3: Incorrectly connecting to な-adjectives or nouns
As mentioned in the structure section, な-adjectives and nouns usually take 「だ」 before 「に違いない」 in their plain form. Omitting it can sometimes make the sentence sound less formal or less complete.
❌ 彼女は綺麗に違いない。（話し言葉ではありえるが、書き言葉ではより丁寧に）
✅ 彼女は綺麗だに違いない。
While the omission of 「だ」 is sometimes heard in casual speech, for a formal N2 expression, it's recommended to stick to the standard plain form with 「だ」 for な-adjectives and nouns. This is especially true in written contexts, as it maintains the clarity and grammatical integrity of the pattern.
Mistake 4: Overusing in casual conversation
Due to its formal and assertive nuance, 「に違いない」 can sound a bit stiff or overly confident if used too frequently in everyday, casual exchanges. Native speakers often opt for softer or more common expressions of probability.
❌ （友達と）今日のランチは美味しいに違いないね！
✅ （友達と）今日のランチは絶対美味しいね！ / 今日のランチは美味しいはずだよ！
While not strictly incorrect, the first sentence might sound a bit strong for a casual lunch with friends. For relaxed settings, using 「絶対(ぜったい)～だ」 or 「～はずだ」 offers a more natural, less formal alternative for expressing strong expectation or certainty.
Cultural Notes
「に違いない」 is a powerful expression of conviction, and its usage reflects a speaker's confidence in their logical deduction. In Japanese communication, where indirectness and softening assertions are sometimes preferred, using 「に違いない」 stands out as a direct and definitive statement. It's often employed in situations where there's a need to present a conclusion with authority, based on solid, observable evidence or strong reasoning.
You'll frequently encounter 「に違いない」 in news reports, academic papers, formal presentations, or mystery novels, where logical deductions are central to the narrative or argument. For example, a journalist reporting on an event might use it to convey their interpretation of facts, or a scientist might use it to express a strong hypothesis based on experimental results. When a detective in a story declares 「犯人は彼に違いない！」 (The culprit must be him!), it signals a breakthrough based on collected evidence, not just a random guess.
In everyday conversations, particularly among friends or family, speakers often opt for slightly softer expressions of probability or certainty. Examples include 「～はずだ」 (it should be/it's expected to be) or even a very confident 「～でしょう」 (it's probably...).
However, if someone uses 「に違いない」 in a casual setting, it often signifies that they feel genuinely certain about their statement. It implies they've considered it deeply or possess compelling evidence to back it up.
It's also worth noting that while 「に違いない」 conveys objective certainty, it is still the speaker's judgment. Therefore, it can sometimes be used to infer someone else's feelings or intentions. However, always understand that this is an interpretation of their actions or words, not a direct statement of their inner state. For example, 「彼は寂しいに違いない」 (He must be lonely) is a strong inference based on his behavior.
JLPT Tips
For the N2 JLPT, mastering 「に違いない」 involves not only understanding its core meaning but also being able to differentiate it from similar expressions of probability and certainty.
Here are some key tips to help you:
Focus on the degree of certainty: 「に違いない」 represents a very high degree of certainty, almost absolute, stemming from logical deduction or strong evidence. When you see a problem asking for the most certain conclusion, this pattern is often the answer. Look for keywords in the context that suggest strong evidence (e.g., 「～から」 because, 「～によると」 according to, 「～からして」 judging from).
Pay close attention to the connection rules: Remember that 「に違いない」 connects to the plain form of verbs and い-adjectives. For な-adjectives and nouns, the standard plain form includes 「だ」. Incorrect conjugation is a common trap in the JLPT. Practice recognizing and forming sentences with all four word types correctly.
Distinguish from similar grammar points: The JLPT frequently tests your ability to choose the most appropriate expression based on nuance. Create a mental hierarchy of certainty for these patterns: に違いない / に決まっている (most certain inference) > はずだ (strong expectation) > だろう / でしょう (general probability) > ようだ / みたいだ / らしい (appearance / hearsay). Understanding the source of the information (direct observation, rumor, logical deduction) is crucial for making the right choice.
Practice context-rich sentences: Don't just memorize definitions. Work with example sentences that provide a clear context or reason for the certainty. For instance, if a sentence describes someone yawning repeatedly, 「彼は眠いに違いない」 (He must be sleepy) is a logical deduction. If it simply states 「彼は眠い」, it's a fact. The nuance lies in the inference.
Consider formality: While the JLPT often focuses on grammatical correctness over conversational nuance, it's good to remember that 「に違いない」 is more formal. This understanding can help you appreciate why certain choices might be more natural in specific contexts, even if multiple options are grammatically sound.