"A big crack"

Translation: Ufa mkubwa

[Kole-Wright]

Why "mkubwa"? I thought this would be "ufa ukubwa".

[Mandiras]

The concord for consonant-stem adjectives is 'm-' in the singular for U/N-class nouns (and 'mw-' for vowel-stem adjectives). The tables in the tips & notes of the 'Adjectives' skill are very useful for reference: https://www.duolingo.com/skill/sw/Adjectives