～に違いない
Be strongly convinced that something is true
You’ll be able to make a confident conclusion when the available facts make one explanation seem almost certain.
いいにおいがするから、隣のテントでバーベキューをしているに違いない。
ii nioi ga suru kara, tonari no tento de baabekyuu o shite iru ni chigainai.
What do you think this sentence means?
Because of the nice smell, the tent next door must be having a barbecue.
Express strong certainty based on evidence
～に違いない expresses the speaker's firm belief or strong conviction that something is definitely true, deduced from objective evidence or circumstances ("must be...", "there is no doubt that...").
あの店はいつも人が並んでいるから、おいしいに違いない。
ano mise wa itsumo hito ga narande iru kara, oishii ni chigainai.
There is always a line at that restaurant, so it must be good.
この字は父の字に違いありません。
kono ji wa chichi no ji ni chigai arimasen.
This handwriting must be my father’s.
Attach に違いない to a plain form or noun
Attach に違いない directly to the plain form of verbs, i-adjectives, na-adjectives (stem without だ), or nouns (without だ).
何日も徹夜で働いたのだから、疲れているに違いない。
nannichi mo tetsuya de hataraita no da kara, tsukarete iru ni chigainai.
Since they worked all night for days, they must be exhausted.
玄関に知らない靴がある。お客さんが来ているに違いない。
genkan ni shiranai kutsu ga aru. okyakusan ga kite iru ni chigai nai.
There are shoes I don’t recognize at the entrance. We must have a guest.
犬がドアの前でしっぽを振っている。兄が帰ってきたに違いない。
inu ga doa no mae de shippo o futte iru. ani ga kaette kita ni chigai nai.
The dog is wagging its tail in front of the door. My brother must have come home.
窓のガラスが割られているから、犯人はそこから入ったに違いない。
mado no garasu ga wararete iru kara, hannin wa soko kara haitta ni chigai nai.
The window glass has been broken, so the culprit must have got in that way.
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