Пока дети катались с горки на санках, бабушка грелась в машине.

Here пока means while.

It introduces a time clause showing that two actions were happening at the same time:

• Пока дети катались... = While the children were sledding...
• бабушка грелась в машине = grandma was keeping warm in the car

A useful thing to know is that пока can mean different things in different contexts, such as for now or bye, but in this sentence it clearly means while.

Because Russian separates the subordinate clause from the main clause with a comma.

The structure is:

• Пока дети катались с горки на санках = subordinate time clause
• бабушка грелась в машине = main clause

So the comma works like English in a sentence such as:

• While the children were sledding, grandma was keeping warm in the car.

Because дети is the subject of катались.

In Russian, the subject of a sentence is usually in the nominative case.

• дети = children
• катались = were riding / sledding / sliding

So дети катались means the children were sledding.

Also note that дети is an irregular plural form of ребёнок (child).

Катались is the natural verb here because it means something like were riding for fun, were going for a ride, or in this context were sledding.

Why not the others?

• катались = they were riding/sliding around, enjoying the activity
• катали = they were rolling something or taking someone/something for a ride
• ехали = they were going/traveling by vehicle or transport, focusing more on movement from one place to another

With sledding, Russian often uses кататься на санках for the activity itself.

So:

• дети катались на санках = the children were sledding
• not just traveling somewhere on sleds, but engaged in the pastime

The -сь is the reflexive ending.

The basic verb is:

• катать / катить in some uses = to roll, to take someone for a ride
• кататься = to ride, to go riding, to enjoy riding

So кататься is the reflexive form and often describes the subject doing the activity themselves.

In this sentence:

• катались = they were riding / sledding

This is very common in Russian:

• кататься на велосипеде = to ride a bicycle
• кататься на лыжах = to ski
• кататься на коньках = to skate
• кататься на санках = to sled

Because с горки means down from the hill.

• с
  • genitive often means from
• горки is genitive singular of горка

So:

• с горки = from/down the hill
• на горке = on the hill

That difference matters here. The children are not just located on the hill; they are sliding down it.

Also, горка often means a small hill or specifically a hill used for sledding, not necessarily a big mountain.

Because the preposition с here means from/down from, and in that meaning it normally takes the genitive.

So:

• с горки = from the hill / down the hill

This is a standard pattern:

• с крыши = from the roof
• с лестницы = from the stairs
• с горки = from the hill

Санках is in the prepositional case plural after на.

The phrase кататься на санках literally means to ride on sleds, but in natural English we usually say to sled or to go sledding.

Important vocabulary point:

• санки is a plural-only noun in Russian
• there is normally no everyday singular form used in this meaning

So you learn it as:

• nominative: санки
• prepositional: на санках

Similar expressions:

• на лыжах = on skis
• на коньках = on skates
• на санках = on a sled / sledding

Even though English often says on a sled, Russian uses the plural noun санки.

In normal modern usage, yes, санки is generally treated as a plural-only noun.

That is why you get forms like:

• санки
• санок
• санкам
• санками
• санках

So in this sentence:

• на санках = on a sled / sledding

For a learner, it is best to memorize санки as a plural noun, much like some nouns in English that mostly appear in plural form, such as scissors or pants.

Грелась is the past tense feminine singular of греться, which means to warm oneself, to get warm, or to keep warm.

So:

• бабушка грелась в машине means that grandma was in the car getting warm or keeping warm there

It does not necessarily mean she was actively heating the car. It describes her state/activity: she was staying warm inside it.

The non-reflexive verb is:

• греть = to warm something

Compare:

• Бабушка грела чай. = Grandma was heating the tea.
• Бабушка грелась в машине. = Grandma was warming herself / keeping warm in the car.

That ending shows two things at once:

1. -л- marks past tense
2. -ась shows feminine singular past reflexive form

Compare:

• грелся = masculine singular
• грелась = feminine singular
• грелось = neuter singular
• грелись = plural

Since бабушка is feminine singular, the verb must agree with it:

• бабушка грелась

Because в meaning in for location takes the prepositional case.

• машина = car
• в машине = in the car

This answers the question where?

Compare:

• в машине = in the car
• в доме = in the house
• в парке = in the park

If there were motion into the car, Russian would usually use the accusative instead:

• в машину = into the car

But here grandma is already inside, so we use в машине.

Because the sentence describes two ongoing actions happening at the same time in the past.

• катались = were sledding
• грелась = was keeping warm

This is exactly the kind of situation where Russian often uses imperfective verbs: background actions, ongoing processes, or repeated activity.

With пока meaning while, imperfective verbs are especially natural when both actions are simultaneous.

In English, this often matches was/were + -ing.

You could translate it literally that way to understand the grammar, but it would not be natural English.

A more natural translation would be:

• were sledding down the hill
• were sliding down the hill on a sled
• were sledding on the hill depending on context

Russian often expresses this more literally than English:

• катались с горки на санках = literally something like rode/slid from the hill on sleds
• natural English: went sledding down the hill

Not exactly.

• гора = mountain
• горка = little hill, mound, or small slope

The ending -к- here gives a diminutive sense. In everyday Russian, горка often refers to the kind of hill children sled down.

So с горки in this sentence strongly suggests a small snowy hill or sledding slope, not a mountain.

The word order can be changed, although the original order is very natural.

Original:

• Пока дети катались с горки на санках, бабушка грелась в машине.

You could also say:

• Бабушка грелась в машине, пока дети катались с горки на санках.

The meaning stays basically the same: the two actions happened simultaneously.

Inside the clauses, word order can also shift a bit for emphasis, but the original sounds neutral and natural.

Because Russian has no articles like a, an, or the.

So:

• бабушка can mean grandma, the grandma, or sometimes a grandmother, depending on context
• в машине can mean in the car or in a car

The exact meaning is understood from the situation. In this sentence, English naturally uses the children, grandma, and the car, but Russian does not need articles to express that.

It is a sentence with a time clause plus a main clause:

• Пока дети катались с горки на санках
  = While the children were sledding down the hill

• бабушка грелась в машине
  = grandma was keeping warm in the car

So the full structure is:

While X was happening, Y was happening.

This is a very common Russian pattern:

• Пока я работал, он читал. = While I was working, he was reading.
• Пока шёл дождь, мы сидели дома. = While it was raining, we stayed at home.