На полдник я купил творожный сырок и кефир.

Here на does not mean a literal physical on. In expressions like на завтрак, на обед, на ужин, на полдник, it means something like for:

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner
• на полдник = for an afternoon snack

So На полдник я купил... means I bought ... for an afternoon snack.

This is a very common Russian pattern: на + accusative to express the purpose of food or a meal occasion.

Полдник is a light meal or snack eaten in the afternoon, often between lunch and dinner. It is especially common in discussions of children's routines, schools, kindergartens, and traditional daily meal patterns.

So it is not exactly the same as a full lunch or dinner. A good English gloss is:

• afternoon snack
• light afternoon meal

Because the expression на полдник uses the accusative case.

The noun полдник is masculine and inanimate. For masculine inanimate nouns, the accusative singular usually looks the same as the nominative singular:

• nominative: полдник
• accusative: полдник

So the form does not change, even though the case does.

Compare:

• Что это? — полдник. = What is this? — An afternoon snack.
• На что? — на полдник. = For what? — For an afternoon snack.

Купил is the past tense of купить, which is a perfective verb meaning to buy in the sense of a completed action.

So я купил means:

• I bought
• I have bought (depending on context)

The ending -л marks past tense masculine singular, which tells us the speaker is male.

Compare:

• я купил = I bought (male speaker)
• я купила = I bought (female speaker)
• мы купили = we bought

Because купить is perfective, the sentence presents the purchase as a finished event.

Yes, but it would change the nuance.

• я купил = I bought it / I ended up buying it
  
  • completed, single event
• я покупал = I was buying / I used to buy / I bought (with focus on process or repeated action)

In this sentence, купил is the natural choice if you simply mean that you went and bought these things for the snack.

If you said На полдник я покупал творожный сырок и кефир, it would sound more like you are emphasizing the process, habit, or background context, which is less natural here without additional explanation.

They are the direct objects of купил, so they are in the accusative case.

However, both happen to look unchanged:

• творожный сырок: masculine inanimate singular
• кефир: masculine inanimate singular

For masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: творожный сырок

• accusative: творожный сырок

• nominative: кефир
• accusative: кефир

That is why the forms do not visibly change.

Творожный сырок is a specific product. It is usually a small sweet curd-cheese bar or snack made from творог.

Breaking it down:

• творог = quark / curd cheese / farmer cheese
• творожный = made from curd cheese
• сырок = literally a small cheese item; in real usage, often a curd snack bar

So творожный сырок refers to the snack itself, not just plain творог.

In everyday Russian, сырок can sometimes be used by itself when the context is obvious, but творожный сырок is the fuller, clearer form.

Yes, творожный is an adjective meaning curd-based or made from curd cheese.

It agrees with сырок in:

• gender: masculine
• number: singular
• case: accusative (same form as nominative here because it is masculine inanimate)

Its dictionary form is творожный. The ending -ый is the normal masculine singular nominative ending for many hard-stem adjectives.

Compare the agreement:

• творожный сырок — masculine singular
• творожная масса — feminine singular
• творожное печенье — neuter singular
• творожные сырки — plural

Кефир is kefir, a fermented milk drink. It is very common in Russian-speaking countries and is a normal everyday food item.

For learners, the main language point is that кефир is a masculine noun:

• этот кефир = this kefir
• я купил кефир = I bought kefir

So in this sentence it is simply the second thing the speaker bought.

Russian normally does not use articles like a, an, and the.

So:

• я купил творожный сырок can mean I bought a curd-cheese bar or I bought the curd-cheese bar, depending on context.
• я купил кефир can mean I bought kefir or I bought some kefir.

English requires articles or determiners much more often than Russian does. In Russian, specificity is usually understood from context, word order, or the situation.

Yes, that is completely possible.

Russian word order is more flexible than English because case endings help show grammatical roles. These versions are all possible:

• На полдник я купил творожный сырок и кефир.
• Я купил на полдник творожный сырок и кефир.
• Творожный сырок и кефир я купил на полдник.

The difference is mainly focus and emphasis:

• На полдник... puts the meal occasion first: As for the afternoon snack...
• Я купил... starts with the subject: more neutral in some contexts
• Творожный сырок и кефир... emphasizes what was bought

The original sentence sounds natural if the speaker wants to highlight that these items were bought specifically for the afternoon snack.

Because творожный сырок и кефир is just a simple list of two items joined by и = and.

In Russian, as in English, you normally do not put a comma before и when it connects two words or two simple items in one phrase:

• яблоки и груши
• чай и кофе
• творожный сырок и кефир

A comma would only appear in more complex structures, not in a basic two-item list like this.

Sometimes in translation, yes, depending on context. But the most direct meaning here is for an afternoon snack.

Russian на + meal word often expresses intended purpose:

• Я сделал бутерброды на завтрак. = I made sandwiches for breakfast.
• Мы купили фрукты на ужин. = We bought fruit for dinner.

So in your sentence, the most natural interpretation is that the speaker bought these foods to have as the afternoon snack.

Yes, it is natural Russian.

It sounds especially normal in contexts such as:

• talking about daily meals
• discussing what someone bought to eat later
• family routines
• school or children's meals
• simple everyday narration

The only cultural note is that полдник is more common in some contexts than in modern casual English meal vocabulary, so the concept may feel more familiar in Russian than in English. But the sentence itself is perfectly ordinary.