Я купил хлеб, кефир и ещё и ватрушку к чаю.

Купил is the past tense, masculine singular form of купить (to buy, perfective).

Why perfective? Because the sentence presents the shopping as a completed result: the speaker bought the items and that action is finished.

Compare:

• Я купил хлеб. = I bought bread. / I have bought bread.
• Я покупал хлеб. = I was buying bread / I used to buy bread / I bought bread (with focus on the process or fact, not the result).

In this sentence, the speaker is listing what they ended up buying, so купил is the natural choice.

Because these nouns are in the accusative case, and different noun types change differently.

After купил, the things bought are direct objects, so they go into the accusative.

What happens here:

• хлеб → хлеб
• кефир → кефир
• ватрушка → ватрушку

Why?

• хлеб and кефир are masculine inanimate singular nouns.
  For these, accusative = nominative, so the form does not change.
• ватрушка is a feminine singular noun ending in -а.
  In the accusative singular, -а usually changes to -у.

So:

• ватрушка = nominative
• ватрушку = accusative

Чаю is dative singular.

It appears after the preposition к, which normally requires the dative case.

So:

• чай = nominative
• чаю = dative

This is a very common pattern:

• к чаю = for tea / to go with tea
• к кофе = for coffee / to go with coffee
• к супу = for the soup / to go with the soup

So чай changes to чаю because of к.

К чаю is an idiomatic expression that usually means for tea, with tea, or to have with tea.

In this sentence, it does not literally mean movement toward tea. Instead, it means the speaker bought the pastry to eat with tea.

So:

• Я купил ватрушку к чаю.
  = I bought a pastry for tea / to have with tea.

This is a very natural Russian way to talk about food that accompanies a drink or meal.

Because they do different jobs.

The first и

This is the normal and joining the list:

• хлеб, кефир и ... ватрушку

The combination ещё и

This is a common expression meaning something like:

• also
• in addition
• sometimes even

So the structure is:

• хлеб, кефир и ещё и ватрушку
• literally: bread, kefir, and also a pastry

The sentence could be simpler without this extra emphasis:

• Я купил хлеб, кефир и ватрушку к чаю.

But ещё и adds the feeling of and on top of that, a vatrushka too.

Ещё и adds extra emphasis.

It suggests that the last item is one more thing besides the expected ones, and sometimes it can sound a little like:

• and also
• and even
• and on top of that

So:

• Я купил хлеб, кефир и ватрушку к чаю.
  Neutral list: I bought bread, kefir, and a pastry for tea.

• Я купил хлеб, кефир и ещё и ватрушку к чаю.
  More expressive: I bought bread, kefir, and also a pastry for tea.

Depending on context, it may sound pleasantly explanatory, slightly proud, or mildly emphatic.

Russian has no articles like a/an/the.

So хлеб, кефир, and ватрушку can mean:

• bread / the bread / some bread
• kefir / the kefir / some kefir
• a vatrushka / the vatrushka

The exact meaning depends on context.

In this sentence, English naturally uses:

• bread
• kefir
• a vatrushka / a pastry

But Russian does not need separate words for that.

Yes, Russian often can omit the subject pronoun when it is clear from the verb.

So both are possible:

• Я купил хлеб, кефир и ещё и ватрушку к чаю.
• Купил хлеб, кефир и ещё и ватрушку к чаю.

Because купил already shows:

• past tense
• singular
• masculine

the listener can often understand that the speaker means I from context.

Why include Я then?

Including Я can:

• make the sentence more explicit
• add slight emphasis to I
• sound more natural if the speaker is contrasting themselves with someone else

So Я is not wrong at all; it is just more explicit.

No, Russian word order is fairly flexible, but different orders sound slightly different in emphasis.

The given sentence is natural:

• Я купил хлеб, кефир и ещё и ватрушку к чаю.

But Russian could also say:

• Я купил к чаю хлеб, кефир и ещё и ватрушку.
• Хлеб, кефир и ещё и ватрушку к чаю я купил.

These alternatives change the focus or style, not the basic meaning.

The original version is straightforward and conversational:

1. speaker
2. action
3. list of things
4. final phrase к чаю

That makes it easy to process.

Because in Russian, as in normal English, you usually do not put a comma before the final and in a simple list.

So:

• хлеб, кефир и ещё и ватрушку

This is standard punctuation:

• comma between earlier items
• no comma before the final и

A comma before the last и would usually only appear in special cases, not in a basic list like this.