must be; I'm sure that

にちがいない states strong conviction that something is true, based on reasoning. Attach it to a plain-form verb/adjective or a bare noun. It is more certain than かもしれない or だろう, but is the speaker's judgment, not proven fact.

Examples

• 電気でんきが消きえている。もう寝ねたにちがいない。

  Denki ga kiete iru. Mou neta ni chigainai.

  The lights are off. They must have gone to bed.

• これは彼かれの傘かさにちがいない。

  Kore wa kare no kasa ni chigainai.

  This must be his umbrella.

Vocabulary in these examples

• 電気 electricity; light
• 消える to go out; to disappear
• いる to exist (animate); to be
• もう already; more
• 寝る to sleep; to go to bed
• これ this

Related patterns · Guessing and hearsay

• 〜そうだ（様態） looks like; seems (from appearance)
• 〜そうだ（伝聞） I hear that; they say that
• 〜ようだ it seems; it appears (based on evidence)
• 〜らしい seems; apparently (from what one heard)
• 〜かもしれない might; maybe