Learn Hardcore Russian: Я купил несколько книг для твоего семинара.

Russian verbs come in pairs of imperfective and perfective aspect. Купил is the perfective past, showing a completed action: “I bought (and finished buying) those books.” Покупал is imperfective, which would mean “I was buying” or “I used to buy,” focusing on the process or habitual past without implying completion.

Why does несколько require the genitive case, giving несколько книг rather than несколько книги or несколько книга?

The quantifier несколько (“several/few”) always governs the genitive plural of the noun that follows. That’s a fixed rule: after несколько, you decline the noun in genitive plural, hence книг.

Why is книг in genitive plural rather than nominative книги or another case?

Because of the combination with несколько, the noun must be in genitive plural. This is sometimes called a “partitive genitive” when expressing an indefinite quantity. So несколько (чего?) — книг.

Why is the phrase для твоего семинара in genitive case? Wouldn’t “для” use the dative?

In Russian, the preposition для always requires the genitive case, not the dative. It conveys “for the benefit or use of.” So we ask для чего? чего? (genitive) — для семинара, and the possessive pronoun твой also switches to genitive masculine: твоего.

How do you decline твой in твоего семинара?

“Семинар” is a masculine, inanimate noun. In the genitive singular, it becomes семинара, and the possessive pronoun твой follows that pattern:

Nominative: твой семинар
Genitive: твоего семинара

Could I say Я купил несколько книг на твой семинар instead of для твоего семинара?

You can, but the nuance shifts. На семинар usually means “for attending the seminar” (ticket, fee, plane ticket). Для семинара emphasizes “for use in the seminar”—materials, handouts, books. In most contexts where you buy books to use during the seminar, для is more natural.

Can I change the word order, for example: Для твоего семинара я купил несколько книг?

Yes. Russian is fairly flexible, so fronting Для твоего семинара for emphasis is perfectly fine. You’d still keep the same cases and agreement; you’re just changing which part of the sentence you highlight.