Я купил свежий батон и кефир в супермаркете.

No. Russian батон is a food word, not the English baton used for a stick or club.

In everyday Russian, батон usually means a long loaf of white bread. It is more specific than хлеб, which means bread in general.

So свежий батон means something like a fresh loaf of white bread.

Купил is the past tense form of купить — to buy.

The form купил tells you:

• it happened in the past
• the subject is singular
• the speaker is male

So:

• Я купил = I bought — said by a man
• Я купила = I bought — said by a woman
• Мы купили = we bought

Also, купить is perfective, so купил usually means a completed action: the buying was successfully done.

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

But in Russian, inanimate masculine singular nouns often have the same form in the accusative as in the nominative.

So:

• nominative: батон, кефир
• accusative: батон, кефир

That is why they look unchanged.

If the noun were feminine, or animate masculine, you would often see a clearer change.

Свежий means fresh and it agrees with батон.

Since батон is:

• masculine
• singular
• inanimate

the adjective is also masculine singular, and in this sentence its accusative form is the same as the nominative form:

• свежий батон

If the noun changed, the adjective would change too:

• свежая булка — a fresh bun/loaf
• свежее молоко — fresh milk
• свежие овощи — fresh vegetables

In this sentence, свежий most naturally describes батон only.

So the meaning is:

• a fresh loaf and kefir

not necessarily:

• a fresh loaf and fresh kefir

If you want to make it clear that both are fresh, the safest natural way is to repeat the adjective:

Я купил свежий батон и свежий кефир в супермаркете.

Because в супермаркете means in/at the supermarket — a location.

After в, Russian uses:

• prepositional case for location: в супермаркете
• accusative case for motion into a place: в супермаркет

Compare:

• Я купил кефир в супермаркете. — I bought kefir in the supermarket.
• Я пошёл в супермаркет. — I went to the supermarket.

Russian has no articles like English a/an and the.

So Я купил свежий батон и кефир в супермаркете can mean:

• I bought a fresh loaf and kefir in a supermarket
• I bought the fresh loaf and the kefir in the supermarket

The exact meaning depends on context.

Russian usually shows definiteness through:

• context
• word order
• intonation
• sometimes extra words like этот for this

No, Russian word order is fairly flexible.

This sentence uses a very neutral, natural order:

Я купил свежий батон и кефир в супермаркете.

But you can move parts around for emphasis:

• В супермаркете я купил свежий батон и кефир. — emphasizes where
• Кефир и свежий батон я купил в супермаркете. — emphasizes what

So Russian word order is not as rigid as English word order, although some orders sound more neutral than others.

Sometimes, yes.

Russian often omits the subject pronoun when it is clear from context. So in conversation, you might hear:

Купил свежий батон и кефир в супермаркете.

However, in the past tense, the verb shows gender and number, but not person as clearly as present-tense forms do. Because of that, keeping Я can make the sentence clearer, especially for learners or in standalone sentences.

So:

• Я купил... = clear and neutral
• Купил... = possible, more context-dependent

This is an aspect question.

• купил comes from купить — perfective
• покупал comes from покупать — imperfective

In this sentence, купил is used because the action is presented as completed: the speaker successfully bought the items.

Very roughly:

• Я купил батон и кефир. — I bought a loaf and kefir. The result matters.
• Я покупал батон и кефир. — I was buying / used to buy / bought, depending on context; it focuses more on the process or repeated action.

For a simple finished event like this, купил is the normal choice.