Questions & Answers about Анна купила молоко и сыр.
Why is купила used here instead of the imperfective form?
Купила is the perfective form of the verb to buy. We use it here because Anna completed the action just one time, and there is a clear result: the shopping is done, and she now has the groceries. If she was in the middle of buying them or bought them regularly, we would use the imperfective form покупала.
Why does the verb купила end in -а?
In the Russian past tense, verbs must match the gender of the person who did the action. Since Анна is feminine, we use the feminine ending -а. If a man had bought the food, the verb would be купил.
Shouldn't молоко and сыр change their endings because they are the objects being bought?
They are in the accusative case (the object case) here! However, for neuter words like молоко and inanimate masculine words like сыр, the accusative form looks exactly the same as the dictionary form. Only feminine words ending in -а or -я get an obvious ending change in this situation.