Learn Hardcore Russian: В аптеке можно купить лекарство.

Because the preposition в when indicating location (“in” somewhere) governs the prepositional case. The base noun аптека (pharmacy) becomes аптеке to show “in the pharmacy.”

What role does можно play in this sentence?

можно is an impersonal modal word meaning “it is possible” or “one can.” It doesn’t change with person or number and introduces the idea that buying medicine is allowed or possible.

Why is the verb купить in the infinitive rather than a conjugated form?

After impersonal modals like можно, Russian uses the infinitive of the main verb. There is no explicit subject, so you don’t say “you buy” or “I buy,” but simply “to buy” (купить) to express the potential action.

Why is лекарство singular here? Can I talk about multiple medicines?

лекарство often functions as a mass noun (“medicine” in general), and is used in the singular. If you want to emphasize distinct items or different kinds, you can say лекарства (“medicines”) or more precisely лекарственные препараты. For counting pills you’d typically specify таблетки (tablets).

Russian has no articles like “a” or “the.” How do we know if it’s “a medicine” or “the medicine”?