Бабушка села на табуретку и начала чистить яблоки для пирога.

Because села is from сесть, the perfective verb meaning to sit down. It describes the action of moving into a sitting position.

• села = sat down
• сидела = was sitting / sat in the sense of already being seated

So in this sentence, the grandmother first sat down, and then she began peeling apples.

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

Since бабушка is a feminine singular noun, the past-tense verbs are also feminine singular:

• сел → masculine
• села → feminine
• село → neuter
• сели → plural

So:

• Бабушка села
• Бабушка начала

both show that the subject is feminine singular.

Because села involves movement onto something. With на, Russian uses:

• accusative for motion toward / onto
• prepositional for location

So:

• села на табуретку = sat down onto a stool
• сидела на табуретке = was sitting on a stool

That contrast is very common in Russian:

• на стол = onto the table
• на столе = on the table

Because it is in the accusative case.

Табуретка is a feminine noun ending in -а. For many feminine nouns like this, the accusative singular changes -а to -у:

• табуретка → nominative
• табуретку → accusative

This happens because the verb села requires на + accusative when there is motion.

After verbs like начать (to begin), Russian usually prefers the imperfective infinitive, because the focus is on the start of a process, not on its completion.

So:

• начала чистить яблоки = began peeling apples

This is the normal pattern:

• начал читать
• начала готовить
• начали писать

You may sometimes see a perfective infinitive after начать, but imperfective is the standard and most natural choice in sentences like this.

In everyday Russian, чистить can mean to clean, but with foods like potatoes or apples it often means to peel.

So here чистить яблоки means something like:

• to peel apples
• or more generally to prepare the apples by peeling them

The exact English wording depends on context, but in a cooking sentence, peel is usually the best translation.

Because яблоки is the normal accusative plural form for an inanimate direct object, and for inanimate plural nouns, the accusative usually looks the same as the nominative.

• nominative plural: яблоки
• accusative plural: яблоки

So чистить яблоки = to peel apples

The form яблок is genitive plural, which is used in other situations, for example after numbers or quantity words:

• много яблок
• пять яблок

Because для takes the genitive case.

The noun пирог becomes пирога in the genitive singular:

• пирог → nominative
• пирога → genitive

So:

• для пирога = for a pie / for the pie

This phrase shows purpose: the apples are being prepared for making a pie.

Russian has no articles like a and the, so the sentence itself does not explicitly mark that difference.

Context tells you which is meant:

• для пирога could mean for a pie
• or for the pie

In many learning translations, for a pie is the most natural choice unless earlier context has already introduced a specific pie.

This word order is natural and neutral for narration:

Бабушка села на табуретку и начала чистить яблоки для пирога.

Russian word order is more flexible than English, because case endings show grammatical relationships. So you can move parts around for emphasis, but the original order sounds like a normal, straightforward description of events.

For example, the sentence first gives:

1. what she did first — села на табуретку
2. what she did next — начала чистить яблоки для пирога

So the order matches the flow of the action very naturally.