に違いないMust be; surely
に違いない expresses the speaker's strong conviction that a conclusion must be true, usually after considering some evidence. It shows a confident judgment, not a fact that is guaranteed to be correct.
A confident conclusion from clues
Use this form when clues lead you to one conclusion and you feel that another explanation is unlikely. English “must” often works well, provided it means a confident inference rather than an obligation.
電気が消えていて、返事もない。もう寝たに違いない。
The lights are off, and there is no answer. They must have gone to bed already.
AnalyzeThe unlit room and lack of a response provide the clues; the final sentence presents the speaker's confident conclusion.
Put tense and negation before the expression
The part before に違いない carries the actual time and positive or negative meaning. Use a past form for “must have done” and a negative form for “must not” or “cannot be the case that.”
財布は机にも鞄にもない。家に置いてきたに違いない。
My wallet is neither on the desk nor in my bag. I must have left it at home.
AnalyzeThe past form appears before the target.
何度電話しても出ない。まだこの知らせを知らないに違いない。
There is no answer no matter how many times I call. They must not know this news yet.
AnalyzeThe negative form appears before the target.
Choosing the right degree of confidence
に違いない foregrounds your strong conviction in a conclusion. はずだ instead emphasizes an expectation based on known facts, a schedule, a rule, or the circumstances. ようだ is a more cautious evidence-based inference, while かもしれない only presents a possibility. はずがない strongly rejects a possibility rather than affirming a conclusion.
More examples
ドアに鍵がかかっている。彼はもう出かけたに違いない。
The door is locked. He must have gone out already.
Analyze部屋は暗く、物音もしない。今は誰もいないに違いない。
The room is dark, and there is not a sound. No one can be there now.
Analyze空が急に暗くなってきた。もうすぐ雨が降るに違いない。
The sky has suddenly grown dark. It must be going to rain soon.
Analyzeこれだけ駅に近ければ、家賃は高いに違いない。
If it is this close to the station, the rent must be high.
Analyzeメールの署名を見るかぎり、新しい担当者は森さんに違いない。
Judging from the email signature, the new person in charge must be Ms. Mori.
Analyzeこの筆跡は以前の手紙と同じです。書いたのは同じ人物に違いありません。
This handwriting is the same as in the earlier letter. It must have been written by the same person.
Analyze