На завтрак я купила банан и грушу.

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

• я купила = I bought (said by a woman)
• я купил = I bought (said by a man)

So this sentence tells you that the speaker is female.

Here на завтрак means for breakfast.

So the idea is:

• На завтрак я купила банан и грушу = I bought a banana and a pear for breakfast

Even though на often means on or onto, in expressions with meals it commonly means for:

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner

This is a very common Russian pattern.

Because на завтрак is the normal, idiomatic way to say for breakfast in this context.

Russian often uses на + accusative when something is intended for a purpose or occasion:

• купить что-то на ужин = to buy something for dinner
• приготовить что-то на обед = to cook something for lunch

Для завтрака is grammatically possible in some contexts, but here it sounds less natural and less idiomatic.

Because both nouns are direct objects, so they are in the accusative case.

Russian accusative endings depend on the noun type:

• банан is a masculine inanimate noun, and in the singular its accusative is the same as the nominative:

  • nominative: банан
  • accusative: банан

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

  • nominative: груша
  • accusative: грушу

So:

• купила банан
• купила грушу

They are in the accusative case, because they are the things that were bought.

The verb купить takes a direct object:

• купить банан
• купить грушу

That is why the nouns appear in the accusative.

Russian has no articles.

So Russian does not have separate words corresponding exactly to English a/an and the.

That means:

• банан can mean a banana or the banana
• грушу can mean a pear or the pear

Which one is meant depends on the context. In this sentence, English would most naturally say a banana and a pear.

Yes. Russian often drops subject pronouns when the meaning is already clear from the verb form or the context.

So both of these are possible:

• Я купила банан и грушу.
• Купила банан и грушу.

Including я can make the subject clearer or slightly more emphatic. In an isolated example sentence, keeping я is very normal.

Yes. Russian word order is more flexible than English word order.

This sentence could also be said as:

• Я купила на завтрак банан и грушу.
• Банан и грушу я купила на завтрак.

The basic meaning stays the same, but the emphasis changes:

• На завтрак first highlights the breakfast context.
• Я first highlights who bought them.
• Банан и грушу first highlights what was bought.

The original sentence sounds very natural.

Купить / купила is the perfective form, which presents the action as completed.

Here the speaker means that she completed one buying action:

• На завтрак я купила банан и грушу.
  = I bought a banana and a pear for breakfast.

If you used покупала, that would be the imperfective form покупать, which usually suggests process, repetition, or background information:

• Я покупала фрукты каждый день. = I used to buy fruit every day.
• Я как раз покупала фрукты, когда... = I was buying fruit when...

So купила is the normal choice here.

Yes. Both nouns are singular, so the sentence means one banana and one pear.

If you wanted plurals, you would say:

• бананы и груши = bananas and pears

So:

• купила банан и грушу = bought a banana and a pear
• купила бананы и груши = bought bananas and pears

A useful stress pattern is:

• На зАтрак? No — the correct stress is На зАвтрак
• я купилА
• банАн
• и грУшу

So the full sentence is:

На зАвтрак я купилА банАн и грУшу.

A rough English-style approximation would be:

na ZAF-trak ya koo-pee-LA ba-NAN ee GROO-shoo

Of course, the real Russian sounds are a bit different, but the stress placement is important.