Утром я съела банан, а сестра взяла с собой грушу.

Утром is the instrumental singular of утро (morning), but in this kind of expression it functions like an adverb: in the morning.

Russian often uses a noun in the instrumental case to express a time period in a general way:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So Утром я съела банан... means In the morning, I ate a banana...

Съела is the perfective past form of съесть, while ела is the imperfective past form of есть.

Here, perfective is used because the action is presented as completed: the speaker ate the banana as a finished event.

• я ела банан = I was eating / I ate banana(s), with focus on the process or habitual action
• я съела банан = I ate up / finished eating a banana

In this sentence, a single completed action is meant, so съела is the natural choice.

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

Since the subject is я, the verb form depends on whether the speaker is male or female.

• я съел = I ate (male speaker)
• я съела = I ate (female speaker)

So съела tells you the speaker is female.

Yes, банан is in the accusative case, but for many inanimate masculine nouns, the accusative singular looks exactly the same as the nominative singular.

• nominative: банан
• accusative: банан

That is why the form does not change.

Compare:

• Я вижу банан. = I see a banana.
• Я съела банан. = I ate a banana.

Both use the accusative, even though the noun looks unchanged.

Because груша is the direct object of the verb взяла, it must be in the accusative case.

For most feminine nouns ending in -а, the accusative singular changes to -у:

• груша → грушу
• книга → книгу
• мама → маму

So:

• сестра = subject, nominative
• грушу = object, accusative

Взяла с собой means took with her / took along.

• взяла грушу = took a pear
• взяла с собой грушу = took a pear with her

The phrase с собой adds the idea that she took it along when going somewhere.

This is a very common Russian expression:

• Я взял с собой воду. = I took water with me.
• Возьми с собой паспорт. = Take your passport with you.

So in your sentence, the sister did not simply pick up a pear; she took it along with her.

С собой is a reflexive expression meaning with oneself / with him-/her-/myself, depending on the subject.

It refers back to the subject of the clause:

• я взял с собой = I took with me
• она взяла с собой = she took with her
• они взяли с собой = they took with them

In your sentence, the subject is сестра, so с собой means with her.

You would use с ней if you meant with her as a separate person, not reflexively. For example:

• Я взял грушу с ней would not mean the same thing and would sound wrong in this context.

So с собой is the correct fixed expression here.

А often links two clauses with a sense of contrast, comparison, or switching to another topic/person. It does not always mean a strong but, but it often feels like while / whereas / and as for.

Here:

• Утром я съела банан, а сестра взяла с собой грушу.

This compares two people and what each one did:

• I ate a banana,
• while my sister took a pear with her.

If you used и, it would sound more like simple addition:

• ...и сестра взяла с собой грушу = ...and my sister took a pear with her.

Using а is very natural here because the sentence sets the two actions side by side.

Russian word order is more flexible than English word order, because case endings show grammatical roles. But word order still affects focus and style.

The given order is natural:

• Утром sets the time first.
• я съела банан gives the first clause.
• а сестра взяла с собой грушу introduces the contrasting second clause.

Other orders are possible, for example:

• Я утром съела банан...
• Сестра взяла с собой грушу, а я утром съела банан.

These versions are grammatical, but they may place emphasis differently. The original sounds neutral and smooth.

Russian often omits possessives like my, your, or his when the meaning is obvious from context.

So сестра can mean sister, and in context it is often understood as my sister.

Russian speakers usually do not say моя сестра unless they want to emphasize it, clarify it, or contrast it with someone else’s sister.

So:

• а сестра взяла с собой грушу naturally means and my sister took a pear with her

By default, утром most clearly attaches to the whole situation or at least to the first clause, and listeners will often understand that both actions happened in that morning context unless something suggests otherwise.

In a sentence like this, it is very natural to read it as:

• In the morning, I ate a banana, and my sister took a pear with her.

If the speaker wanted to make the timing different for the second clause, Russian would normally make that explicit.

So yes, утром can be understood as setting the time frame for the whole sentence, not just the first action.