К чаю я купил зефир.

Because к normally takes the dative case. The noun чай is the dictionary form, and its dative singular form is чаю.

So:

• чай = tea
• к чаю = for tea / to have with tea

In this sentence, к чаю is an idiomatic expression meaning something like for tea, to go with tea, or to serve with tea.

Historically and grammatically, к often has the idea of movement toward something, but in expressions like к чаю it does not mean physical movement. Here it means for tea in the sense of to eat with tea or for the tea table.

So К чаю я купил зефир means something like:

• I bought marshmallow sweets for tea
• I bought some zefir to have with tea

It is a very natural Russian way to talk about food bought to go with tea.

Зефир is a type of soft sweet popular in Russia and other post-Soviet countries. It is often translated as marshmallow, but it is not exactly the same as the typical American marshmallow.

It is usually:

• soft and airy
• fruit-flavored
• often shaped in round halves
• sometimes coated in chocolate

So if the meaning shown to the learner says marshmallow, that is a helpful approximation, but зефир is really its own specific confection.

Because зефир is the direct object of купил, so it is in the accusative case. However, for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: зефир
• accusative: зефир

That is why the form does not change.

Compare that with an animate masculine noun, where the accusative often matches the genitive instead.

Yes, Russian often can leave it out if the subject is already clear from the verb form.

• Купил already tells you I bought if the context makes the speaker obvious.
• So К чаю купил зефир can be possible in the right context.

But я is included here to make the subject explicit. It can help with:

• clarity
• emphasis
• contrast

For example, К чаю я купил зефир can sound a bit like As for tea, I bought zefir or I bought zefir for tea with mild emphasis on I.

Russian word order is more flexible than English word order. Both are possible, but they can sound slightly different in emphasis.

• Я купил зефир к чаю = a more neutral I bought zefir for tea
• К чаю я купил зефир = puts for tea first, so that idea is highlighted or treated as the topic

Starting with К чаю can suggest:

• this is what the purchase was for
• we are talking about preparations for tea
• the speaker is answering an implied question like What did you buy for tea?

So the sentence begins with the phrase the speaker wants to foreground.

Купил is the past masculine singular form of the perfective verb купить.

Perfective is used because the sentence presents the buying as a completed action:

• купил = bought, successfully completed the purchase

The imperfective partner is покупать:

• Я покупал зефир = I was buying / used to buy / bought (with focus on process or repetition, depending on context)

Here, the speaker is reporting a finished result, so купил is the natural choice.

In the past tense, Russian verbs agree with the subject in gender and number if the subject is singular.

So:

• я купил = I bought, if the speaker is male
• я купила = I bought, if the speaker is female

That means this sentence, as written, suggests that the speaker is male.

Sometimes, but not with exactly the same feel.

• к чаю is the standard idiomatic expression for food or sweets meant to have with tea
• для чая literally means for tea, but it is less idiomatic in this context

Russian speakers very commonly say things like:

• Что купить к чаю? = What should we buy for tea?
• Я взял печенье к чаю. = I got cookies for tea.

So for snacks, desserts, or sweets intended to be eaten with tea, к чаю is usually the most natural choice.

It can suggest both, depending on context.

In Russian, чай can refer not only to the drink but also to the whole idea of having tea, often with something sweet. So к чаю may mean:

• to go with the drink tea
• for tea time
• for serving when people sit down to tea

That is why К чаю я купил зефир sounds very natural: it refers to buying something suitable to eat during tea.

Usually not in any special, strongly definite English sense. Russian does not have articles like a and the, so the sentence itself does not directly mark definiteness the way English does.

зефир here is best understood simply as:

• some zefir
• zefir
• a marshmallow-type sweet

Whether it is specific or not depends on context, not on an article.