Questions & Answers about Я купил книгу для друга.
What is the grammatical structure of the sentence Я купил книгу для друга?
The sentence follows a Subject-Verb-Object pattern with an additional prepositional phrase. Here, Я is the subject (in the nominative case), купил is the verb in the past tense (masculine singular), книгу is the direct object in the accusative case, and для друга is a prepositional phrase where для requires the following noun to be in the genitive case.
Why is the verb form купил used instead of a different ending like купила?
купил is the masculine singular past tense form of купить. In Russian, past tense verbs agree with the speaker's gender. Since the speaker is assumed to be male here, купил is used; a female speaker would say купила.
What case is книгу in, and why is that case appropriate?
книгу is in the accusative case. In Russian, the direct object of an action—especially with verbs like купить (to buy)—must be in the accusative case. For the feminine noun книга, the accusative singular form is книгу.
Why does the phrase для друга use the genitive case?
The preposition для always requires its noun to be in the genitive case. Therefore, друг becomes друга in the genitive. This phrase specifies for whom the book was bought—it indicates the beneficiary of the action.