に違いない【にちがいない】

に違いない says the speaker is sure of a conclusion: "must be", "I'm sure that". It follows the plain form of a verb or an い-adjective, and comes straight after a noun or a な-adjective, with no だ and no な.

• × 有名なに違いない → 有名に違いない
• × 彼だに違いない → 彼に違いない

The past goes on the word in front: みちにまよった道に迷ったにちがいない違いない, "they must have got lost", and ゆうめい有名だったにちがいない違いない in the past for a noun or な-adjective.

に違いない or はずだ

はずだ reasons from what you know – a timetable, a promise, how things work. に違いない is the speaker's own conviction, and it can rest on a hunch.

It is stronger than だろう and much stronger than かもしれない.

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に違いない is more at home in writing than in conversation, where きっときごう〜とおもう思う or ぜったい絶対きごう〜よ is more common. The polite form is にちがい違いありません, and にそういない相違ない is a stiffer written version.