Learn Hardcore Russian: Сколько яблок ты купил?

In Сколько яблок ты купил? the word яблок is in the genitive plural. After the quantity word сколько (“how many”), the counted noun always takes the genitive case rather than the accusative. Many Russian nouns form the genitive plural with a zero ending, so яблок is the correct form for “apples.”

Why is the verb купил in the masculine singular form?

Russian past-tense verbs agree with the subject in gender and number. Here the implicit subject is ты, and if you’re asking a male friend, you use masculine singular купил. If you were asking a female friend, you would say Сколько яблок ты купила?

Can I omit ты in this question?

Yes. Russian is a pro-drop language, so the pronoun ты is optional when the verb ending makes the person clear. You can simply ask:
Сколько яблок купил?
Including ты adds emphasis or clarity but isn’t grammatically required.

How would I make this question more formal or address more than one person?

Replace ты with вы and adjust the verb accordingly. For formal or plural “you,” ask:
Сколько яблок вы купили?

Is the word order in Сколько яблок ты купил? fixed, or can I rearrange it?

Russian has quite flexible word order. For example:

Ты сколько яблок купил? (emphasizes “how many”)
Сколько ты купил яблок? (more neutral)
You can move elements for emphasis without changing the basic meaning.

How do I answer this question in Russian?

You can say:
Я купил «number»«apple(s)».
For example:

Я купил пять яблок.
Купил три яблока.
Notice that after the numeral, the noun form depends on the number (see next question).

Why do nouns change forms after different numbers (e.g. три яблока vs пять яблок)?

Russian uses three patterns with cardinal numbers:
1) 1 → nominative singular
• одно яблоко
2) 2–4 → genitive singular
• три яблока
3) 5 and above → genitive plural
• пять яблок
That’s why the ending shifts between -о, -а, and the zero ending.

How would I ask “How many apples are you buying?” (present tense)?

Switch the verb to present tense of покупать:
Сколько яблок ты покупаешь?
All other rules (case of яблок, optional ты, etc.) stay the same.