に違いない: strong conviction based on clues
Use に違いない to draw a confident conclusion from the circumstances. Distinguish this from an expectation based on known information, a mere possibility, and an obligation to act.
Read the evidence behind the conviction
部屋の電気がついている。誰かいるに違いない。 means that the light is on in the room, so someone must be there. The light is the clue; someone's presence is the conclusion drawn from it.
The expression conveys strong conviction, but that is not the same as direct confirmation. Even if the room later turns out to be empty, に違いない represented the speaker's judgment at that moment.
Connecting forms and the position of negation
Use a plain form with verbs and i-adjectives, and omit だ after nouns and na-adjectives. 彼は専門家に違いない。 means that he must be an expert; このかばんは重いに違いない。 expresses confidence that this bag must be heavy.
彼はまだ知らせを聞いていないに違いない。 means that he must not have heard the news yet. The strong conviction applies to the negative content 聞いていない; the speaker is not expressing weaker confidence.
Comparing はずだ and かもしれない
時刻表では、バスはもうすぐ来るはずです。 is an expectation based on the timetable. Replacing はずだ with に違いない can bring out the speaker's conviction, but the two expressions should not be assigned fixed numerical probabilities.
誰かいるかもしれない。 leaves open the possibility that someone is there. When English must translates に違いない, it expresses an inference, not a rule or obligation requiring someone's presence.
Quick check: Does この字は父の字に違いない。 mean that the speaker's father has to write something?
Show the answer and why
No. It confidently judges this to be the father's handwriting. It does not tell anyone what action they are required to take.