Questions & Answers about Старый шкаф стал пыльным, и мама взяла тряпку.
Стал is the past tense of стать “to become, to turn (into something).”
So Старый шкаф стал пыльным literally means “The old wardrobe became dusty,” focusing on the change of state: it was not dusty before, and then it ended up dusty.
If you said Старый шкаф был пыльным, that would mean “The old wardrobe was dusty” (describing a state, without emphasizing the process of becoming dusty). In this sentence, стал is better because it implies that time passed and the wardrobe gradually got dusty.
Пыльным is the instrumental case of the adjective пыльный (“dusty”).
With verbs of becoming and being (like стать, быть, казаться, становиться) Russian very often uses the instrumental case for predicate nouns and adjectives:
- Он стал врачом. – “He became a doctor.”
- Она была учительницей. – “She was a (female) teacher.”
- Шкаф стал пыльным. – “The wardrobe became dusty.”
So стал пыльным is the standard pattern: стать + adjective in the instrumental = “to become + adjective.”