彼(かの)女(じょ) kanojoは wa約(やく)束(そく) yakusokuを o忘(わす)れ()た() wasuretaに ni違(ちが)い()な()い() chigainai。 She must have forgotten her promise. Literal She [topic-は] promise [object-を] forgot [must be-に違いない]. ~に違いない expresses strong inference — 'there's no doubt that.' 約束を忘れた is 'forgot the promise.' The speaker is confident in their conclusion about her forgetfulness. promises~に違いない (must be)
彼(かの)女(じょ) kanojoは wa約(やく)束(そく) yakusokuを o忘(わす)れ()た() wasuretaに ni違(ちが)い()な()い() chigainai。 She must have forgotten her promise. Literal She [topic-は] promise [object-を] forgot [must be-に違いない]. ~に違いない expresses strong inference — 'there's no doubt that.' 約束を忘れた is 'forgot the promise.' The speaker is confident in their conclusion about her forgetfulness. promises~に違いない (must be)