Я взяла бублик в дорогу, а дома доела слойку с яблоком.

Взяла is the past tense feminine singular form of взять (to take).

• взял = a man took
• взяла = a woman took
• взяло = it took
• взяли = they took

So this sentence tells you the speaker is female.

Why not брала? Because взять is perfective, and брать is imperfective.

• взяла = took (completed action)
• брала = was taking / used to take / took repeatedly, depending on context

Here the speaker is describing two completed actions, so perfective verbs fit naturally:

• взяла = took
• доела = finished eating

Good question. Бублик is a masculine inanimate noun, and in Russian the accusative singular of masculine inanimate nouns is usually the same as the nominative singular.

So:

• nominative: бублик
• accusative: бублик

That is why after взяла you still see бублик.

Compare:

• Я взяла бублик. = I took a bagel.
• Я вижу бублик. = I see a bagel.

But with a feminine noun like слойка, the accusative does change:

• nominative: слойка
• accusative: слойку

So in the sentence:

• взяла бублик
• доела слойку

В дорогу is a very common expression meaning for the trip, for the road, or to take along on the journey.

Literally, it looks like into the road/journey, but in real use it often means something like:

• to take with you on a trip
• as travel food
• for later while traveling

So:

• Я взяла бублик в дорогу = I took a bagel for the trip / to eat on the way.

This is a fixed, natural expression in Russian.

Because в дорогу uses the accusative case, while в дороге uses the prepositional case, and they mean different things.

• в дорогу = for the journey / onto the journey / to take along
• в дороге = on the road / while traveling

So:

• взять что-то в дорогу = to take something along for the trip
• есть что-то в дороге = to eat something while on the road

In your sentence, the idea is that the speaker took the bagel along, so в дорогу is the natural choice.

А often connects two parts that are being contrasted, compared, or set against each other, even if the contrast is mild.

Here:

• Я взяла бублик в дорогу, а дома доела слойку с яблоком.

This creates a contrast between:

• в дорогу = on the trip / for the road
• дома = at home

So the sentence feels like:

• I took a bagel for the trip, whereas / and then at home I finished the apple pastry.

If you used и, it would sound more like a simple list of actions:

• Я взяла бублик в дорогу и дома доела слойку с яблоком.

That is possible in some contexts, but а is more natural when the speaker is balancing two different settings or situations.

Here дома is an adverb, meaning at home.

So:

• дома = at home
• домой = homeward / to home
• дом = house/home (noun)

Examples:

• Я дома. = I’m at home.
• Я иду домой. = I’m going home.

In your sentence:

• а дома доела... = and at home I finished eating...

This дома is not the same as the noun plural дома meaning houses. The meaning is clear from context.

Доела comes from доесть, which means to finish eating what was left.

That prefix до- often gives the idea of:

• bringing something to completion
• finishing the remainder

So:

• ела = was eating / ate
• съела = ate up / ate completely
• доела = finished eating the rest

In this sentence, доела слойку с яблоком suggests that some of the pastry had already been eaten earlier, and then at home the speaker finished the rest.

So the nuance is stronger than just ate.

Because these two parts play different grammatical roles.

1. слойку

This is the direct object of доела, so it goes into the accusative case.

• nominative: слойка
• accusative: слойку

2. с яблоком

This means with apple / apple-filled, and the preposition с here requires the instrumental case.

• nominative: яблоко
• instrumental: яблоком

So:

• доела что? → слойку
• слойку с чем? → с яблоком

That is why the two nouns have different endings.

In natural English, it usually means an apple pastry or a puff pastry with apple filling.

Literally, с яблоком is with apple, but in food descriptions Russian often uses с + instrumental to describe a filling or ingredient:

• пирожок с мясом = a meat pie / meat-filled pastry
• булочка с корицей = a cinnamon bun
• йогурт с клубникой = yogurt with strawberry

So слойка с яблоком is best understood as a pastry containing apple, not necessarily a pastry physically sitting next to a whole apple.

Yes, Russian word order is flexible, but different orders change the emphasis.

Current sentence:

• Я взяла бублик в дорогу, а дома доела слойку с яблоком.

This sounds natural and highlights:

1. the first action
2. then the contrastive setting at home
3. then the second action

You could also say:

• Я взяла в дорогу бублик, а дома доела слойку с яблоком.
• Бублик я взяла в дорогу, а дома доела слойку с яблоком.

These are possible, but the emphasis shifts:

• взяла в дорогу бублик puts more focus on for the trip
• Бублик я взяла... puts special focus on the bagel

So the original word order is just a neutral, natural way to say it.

Yes. The sentence presents two completed, one-time actions:

• взяла = took
• доела = finished eating

That makes the sentence feel like a sequence of finished events.

If you changed them to imperfective forms, the meaning would shift:

• брала could sound habitual, repeated, or process-focused
• доедала could sound like was finishing or used to finish

So the perfective forms are doing important work: they tell you that the speaker is talking about concrete completed actions, not an ongoing process or a habit.