Learn Hardcore Russian: Прохожий купил мороженое у продавца. - The passerby bought ice cream from the vendor.

мороженое is in the accusative case because it’s the direct object of the verb купил. As a neuter noun, its nominative and accusative forms are identical.

Why is продавца in the genitive case after у?

The preposition у always requires the genitive case and here indicates the source or “seller” from whom something is bought: “bought … from the seller.”

Is мороженое really a noun? It looks like an adjective.

It began as a neuter adjective meaning “frozen,” but in modern Russian it’s substantivized—fully functioning as a noun—yet it keeps the adjective declension pattern.

Why is купил in the past tense masculine form?

Russian past‐tense verbs agree in gender and number with their subject. Прохожий is masculine singular, so the verb takes the masculine singular ending -л.

Could we say покупал instead of купил?

Yes, but the meaning shifts by aspect. Купил is perfective (completed action: “did buy”), while покупал is imperfective (process or habitual: “was buying” or “used to buy”).