Я купил спелый персик на рынке.

Because купил is the perfective past form of купить, and it presents the action as completed: the speaker bought the peach.

• Я купил спелый персик = I bought a ripe peach / I did buy a ripe peach
• Я покупал спелый персик would usually suggest a different viewpoint, more like:
  • I was buying a ripe peach
  • I used to buy ripe peaches
  • or a focus on the process rather than the completed result

So in a simple statement about one finished purchase, купил is the natural choice.

In Russian, the past tense is usually formed with -л added to the verb stem.

For купить:

• infinitive: купить
• past masculine singular: купил

Past tense in Russian also agrees with gender and number:

• masculine: купил
• feminine: купила
• neuter: купило
• plural: купили

So Я купил means the speaker is male, or is speaking in masculine form.

A female speaker would say:

• Я купила спелый персик на рынке.

That is because Russian past tense agrees with the gender of the subject.

So:

• male speaker: Я купил
• female speaker: Я купила

This is something English does not do, so it often stands out to learners.

Because персик is the direct object, and it is masculine singular inanimate.

In Russian, masculine singular inanimate nouns in the accusative case look like the nominative:

• nominative: спелый персик
• accusative: спелый персик

That is why both the adjective and noun stay in this form here.

Compare that with a masculine animate noun:

• Я вижу нового студента. Here the accusative looks like the genitive because студент is animate.

So:

• персик = inanimate
• therefore accusative = nominative
• result: спелый персик

Because it is the thing being bought, so it is the direct object of the verb купил.

In Russian, direct objects usually go in the accusative case.

So:

• Я = subject
• купил = verb
• спелый персик = direct object in the accusative

In this sentence, the accusative form happens to look the same as the nominative because персик is masculine singular inanimate.

Because на рынке expresses location: at the market.

After на, Russian often uses:

• prepositional case for location
• accusative case for motion toward something

So:

• на рынке = at the market / in the market
• на рынок = to the market

Examples:

• Я купил персик на рынке. = I bought a peach at the market.
• Я пошёл на рынок. = I went to the market.

Here the sentence describes where the buying happened, so на рынке is correct.

Russian often uses на with places that are thought of as open areas, events, institutions, or public venues, and рынок is commonly one of those words.

So Russian says:

• на рынке = at the market

not usually:

• в рынке

English speakers often want to translate at/in the market word-for-word, but Russian prepositions do not match English exactly. You just learn that рынок normally goes with на when talking about being at the market.

Because after на in the sense of location, the noun goes into the prepositional case.

The noun is:

• dictionary form: рынок

Prepositional singular:

• на рынке

This is a normal case change. Many masculine nouns take -е in the prepositional singular.

So:

• рынок → на рынке

No. Russian does not have articles like English a/an and the.

So Я купил спелый персик на рынке could mean:

• I bought a ripe peach at the market
• I bought the ripe peach at the market

Which one is meant depends on context.

Russian usually shows definiteness through:

• context
• word order
• emphasis
• sometimes other words like этот (this) if needed

Because adjectives in Russian must agree with the noun they describe in:

• gender
• number
• case

Here, персик is:

• masculine
• singular
• accusative (same shape as nominative because it is inanimate)

So the adjective must match:

• спелый персик

If the noun changed, the adjective would change too:

• спелая груша = a ripe pear
• спелое яблоко = a ripe apple
• спелые персики = ripe peaches

Спелый means ripe.

It is commonly used for fruit and other things that ripen:

• спелый персик = ripe peach
• спелый банан = ripe banana
• спелые помидоры = ripe tomatoes

It does not mean mature in every sense. For people, for example, you usually would not use спелый the way English sometimes uses ripe figuratively.

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Я купил спелый персик на рынке.

But other orders are possible, depending on emphasis:

• На рынке я купил спелый персик.
  Emphasizes where it happened.

• Спелый персик я купил на рынке.
  Emphasizes what was bought.

• Персик я купил на рынке.
  Stronger focus on the peach.

Even though the order can change, the original version is a very natural, straightforward sentence.

Sometimes, but not as easily as in the present tense.

Russian often drops pronouns when the meaning is clear, but in the past tense the verb does not show person, only gender and number:

• купил = bought, masculine singular
• it does not by itself mean specifically I bought or he bought

So without context, Я is helpful and often necessary.

For example:

• Купил спелый персик на рынке.
  This can work in conversation if it is already obvious who did it, but by itself it feels incomplete or context-dependent.

So in an isolated sentence, keeping Я is normal.

The stress is:

• Я купи́л спе́лый пе́рсик на ры́нке.

Word by word:

• я
• купи́л
• спе́лый
• пе́рсик
• на
• ры́нке

Stress is very important in Russian because it is not always predictable and can sometimes distinguish forms or words.