На рынке я купил банан и грушу для завтрака.

Russian uses different prepositions for location depending on the noun and the usual way speakers conceptualize the place.

With рынок (market), Russian normally says на рынке when you mean at the market / in the marketplace. This is just the standard idiomatic choice.

So:

• на рынке = at the market
• в магазине = in the store
• на почте = at the post office
• на вокзале = at the station

Even though English often uses in, Russian may use на for certain places.

рынке is in the prepositional case.

After на when it means in/at a place, Russian usually uses the prepositional case:

• рынок → на рынке
• стол → на столе
• работа → на работе

So in На рынке, the noun changes from its dictionary form рынок to рынке because it follows на in a location meaning.

Both банан and грушу are direct objects of купил (bought), so both are in the accusative case.

But Russian nouns change differently depending on gender and animacy.

• банан is a masculine inanimate noun
  For masculine inanimate nouns, the accusative is often the same as the nominative:

  • банан → банан

• груша is a feminine noun ending in -а
  In the accusative singular, -а usually changes to -у:

  • груша → грушу

So:

• я купил банан
• я купил грушу

Because in Russian, some accusative forms are identical to the nominative.

For masculine inanimate singular nouns, accusative = nominative:

• банан → банан
• стол → стол
• телефон → телефон

That means банан is still accusative here, even though it does not visibly change.

This is very common in Russian and can feel strange to English speakers at first.

The preposition для (for) requires the genitive case.

So:

• завтрак → завтрака
• мама → мамы
• брат → брата

That is why the phrase is:

• для завтрака = for breakfast

Whenever you use для, you should expect the noun after it to be in the genitive.

купил is the perfective past form of купить, and it usually means a completed action: bought.

In this sentence, the speaker means that the purchase was completed:

• На рынке я купил банан и грушу...
• At the market I bought a banana and a pear...

If you used покупал, that would be the imperfective past and would sound more like:

• I was buying
• I used to buy
• I bought (with focus on the process or repeated action, depending on context)

Here, the simple completed action is the natural choice, so купил fits best.

Not always. Russian often omits subject pronouns when the verb already makes the subject clear.

So both of these are possible:

• На рынке я купил банан и грушу для завтрака.
• На рынке купил банан и грушу для завтрака.

However, я may be included for clarity, emphasis, or just natural style.

Including я can help if:

• you want to contrast with someone else
• you want to make the subject explicit
• you want the sentence to sound a bit more balanced

So я is not strictly required, but it is perfectly natural.

Yes, Russian word order is fairly flexible.

The sentence starts with На рынке to set the scene first: At the market...

You could also say:

• Я купил на рынке банан и грушу для завтрака.
• Банан и грушу я купил на рынке для завтрака.
• Для завтрака я купил на рынке банан и грушу.

These versions all express roughly the same basic meaning, but the emphasis changes.

In the original sentence:

• На рынке is the setting
• я купил is the main action
• банан и грушу are the things bought
• для завтрака gives the purpose

Russian often moves elements around to highlight what is most important.

Russian has no articles.

So a noun like банан can mean:

• a banana
• the banana

and the exact meaning depends on context.

In this sentence, English naturally uses a banana and a pear, but Russian simply says:

• банан и грушу

You learn whether something is definite or indefinite from the situation, word order, stress, and context—not from articles.

Usually, yes: by default it sounds like one banana and one pear.

Russian singular nouns normally refer to one item unless context suggests otherwise.

So:

• банан = a banana / one banana
• грушу = a pear / one pear

If the speaker meant more than one, Russian would normally use plural forms:

• бананы и груши = bananas and pears

Because банан is a masculine inanimate singular noun in the accusative, and that form is the same as the nominative.

So:

• nominative: банан
• accusative: банан

The form банана is usually genitive singular (and in some contexts accusative for animate nouns, but not here).

So in this sentence, банана would be wrong because the verb купил takes a direct object in the accusative, and банан already is the correct accusative form.

Here для завтрака simply means for breakfast—in other words, the banana and pear are intended to be eaten at breakfast.

Russian often uses для + genitive to show purpose:

• для работы = for work
• для школы = for school
• для завтрака = for breakfast

So the phrase tells you the reason or intended use of what was bought.

The context makes that clear.

In Russian, рынок can mean:

• a physical marketplace / market
• the market in the economic sense

But in На рынке я купил банан и грушу..., the sentence is clearly about a place where someone bought fruit, so it means the market / marketplace.

Also, на рынке is a very common phrase for shopping at an open market or marketplace.