Learn Hardcore Russian: Дети сразу бегут на качели, а я сажусь рядом с горкой на скамейку. - The children immediately run to the swings, and I sit down on a bench next to the slide.

на качели (preposition на
- accusative) = onto the swings / to the swings as a place to get on them
- stresses the target surface/place where the action ends.
- very natural with play equipment, furniture, transport:
  - сесть на стул – to sit on a chair
  - идти на остановку – to go to the bus stop
  - бежать на качели – to run to (onto) the swings
к качелям (preposition к
- dative) = towards the swings (in their direction)
- stresses the direction, not necessarily the final “getting on” them.
- more neutral about what happens when you arrive.

In this sentence, the kids are running specifically in order to get on and use the swings, so на качели is the most natural option.

Why is качели in the plural? Is there a singular form?

Качели is one of those Russian nouns that is practically only used in the plural (pluralia tantum), like ножницы (scissors), часы (a clock / watch).

качели – swings (the whole structure where you sit and swing)

There is a singular form качеля, but:

it sounds bookish, technical or very unusual in everyday speech;
ordinary native speakers almost always use the plural качели, even when talking about a single set of swings.

So you should just learn качели as:

nominative plural: качели
accusative plural (same form here): качели
prepositional: на качелях
…and so on.

Why is it бегут, not бегают? Both come from to run, right?

Yes, both are from the verb of motion to run, but they differ in meaning:

бегут – from бежать (unidirectional, concrete motion)
- describes motion in one concrete direction / trip
- fits “They are running (there) now / in this instance”
бегают – from бегать (multidirectional, repeated / habitual motion)
- describes repeated, habitual, or back-and-forth motion
- fits “They run (around) a lot”, “They used to run here” etc.

In Дети сразу бегут на качели the idea is:

in this particular moment, in this situation, they run (once) from here to the swings,
so the unidirectional бегут is correct.

Why is the verb in the present tense (бегут) if we might translate this as “The children immediately ran to the swings”?

Russian often uses present tense to describe:

habits,
typical actions,
repeated behavior in a certain situation,

even if you’d use the present simple or even past in English, depending on context.

So this Russian sentence can be understood as:

“Whenever we come to the playground, the children immediately run to the swings, and I sit on the bench.”

English might also use present simple here (“run”), but if the wider story is in the past, an English translation could become “ran”. Russian can still stay in the present to describe a typical scene. Context decides the English tense.

What is the difference between сажусь, сижу, and сяду? They all look like to sit.

These three are related but different:

садиться – to sit down, to be in the process of sitting down (imperfective)
- я сажусь – I am sitting down / I (usually) sit down
сесть – to sit down (once, successfully) (perfective)
- я сяду – I will sit down (at some specific moment)
сидеть – to sit, to be in a sitting position (imperfective)
- я сижу – I am sitting / I sit

So:

я сажусь на скамейку – I (start to) sit down on the bench.
я сяду на скамейку – I’ll sit down on the bench (at that moment, as a completed action).
я сижу на скамейке – I am sitting on the bench (already seated).

In the sentence, сажусь focuses on the action of sitting down, not the resulting state.

What does the -сь at the end of сажусь do?

The -сь / -ся ending marks a reflexive verb.

сажусь = сажу + -сь, from садиться
literally, something like “I seat myself”, which in normal English is just “I sit down”.

Without -ся, the verb changes meaning:

сажу (from сажать) – “I seat (someone else) / I plant (something)”
- Я сажу ребёнка на стул. – I sit the child on the chair.
- Я сажу цветы. – I am planting flowers.

With -ся, садиться / сесть means “to sit down (oneself)”. So сажусь is “I am sitting down”.

Why is it рядом с горкой, and why is горкой in that form?

рядом с is a fixed expression:

рядом с
- instrumental case = next to / beside (something)

So the noun after с must be in the instrumental case.

горка (nominative)
горкой (instrumental)

Therefore:

рядом с горкой – next to the slide.

Other examples:

рядом с домом – next to the house
рядом с мамой – next to mom
рядом со школой – next to the school (note the form со before ш for easier pronunciation)

What exactly is a горка here? Does it literally mean a “little hill”?

Literally, yes:

гора – a mountain
горка – a small hill / “little mountain” (diminutive)

But in playground context, горка very commonly means:

a slide (the thing children climb and slide down).

So in this sentence, горка is almost certainly a playground slide, not a natural hill. Context (playground, swings, bench) strongly suggests that meaning.

Why is it на скамейку and not на скамейке?

The preposition на can take either:

accusative – motion onto / to
prepositional – location on / at

Here:

на скамейку (accusative) – onto the bench (movement)
- сажусь на скамейку – I am sitting down onto the bench.
на скамейке (prepositional) – on the bench (static location)
- сижу на скамейке – I am sitting on the bench.

Since сажусь describes the motion of sitting down, accusative (на скамейку) is used.

What is the difference between а я сажусь and и я сажусь here? Why use а instead of и?

Both а and и can be translated as and, but they have different nuances:

и – simple addition: and
- connects similar things without contrast.
а – often implies contrast, comparison, or a shift of focus:
- “while”, “whereas”, “but as for…”, depending on context.

In Дети сразу бегут на качели, а я сажусь…:

а contrasts what the children do with what I do:
- “The children immediately run to the swings, whereas I sit down next to the slide on a bench.”

Using и would sound more neutral and less contrasted; а emphasizes the difference in actions.

Can I change the word order, for example: Я сажусь на скамейку рядом с горкой?

Yes, that is perfectly natural and maybe even more typical word order:

Я сажусь на скамейку рядом с горкой.

Both:

я сажусь рядом с горкой на скамейку
я сажусь на скамейку рядом с горкой

are grammatically correct, but usually:

verb (сажусь),
place you go to (на скамейку),
additional location detail (рядом с горкой)

sounds a bit smoother. Native speakers often say it like that.

What does сразу contribute to the meaning? Is it like “immediately”?

Yes, сразу means immediately / right away / at once.

Дети сразу бегут на качели – The children immediately run to the swings.

Nuances:

сразу – neutral “immediately / right away”.
сразу же – a bit more emphatic: “immediately, right then and there”.

You can place сразу in a few spots, but here it naturally comes before the verb:

Дети сразу бегут на качели.
Putting it after the verb (Дети бегут сразу…) is possible but often changes the rhythm or emphasis.

Why is there a comma before а, and is that always necessary?

Yes, in this sentence the comma is required.

а is a coordinating conjunction connecting two independent clauses:
- Дети сразу бегут на качели (clause 1)
- я сажусь рядом с горкой на скамейку (clause 2)

Russian spelling rules say:

A comma is used before coordinating conjunctions (и, а, но, однако…) when they join two independent clauses.

So:

Дети сразу бегут на качели, а я сажусь рядом с горкой на скамейку.

You would not omit this comma in standard written Russian.

Could the subject я be dropped here, like in some other languages?

In Russian, subject pronouns are usually not dropped unless:

they are very obvious from context and
the sentence sounds natural without them.

Formally, you could say:

…, а сажусь рядом с горкой на скамейку.

A native speaker would understand it, but it sounds slightly incomplete or stylistically odd as a standalone sentence, because:

the previous subject is Дети,
without я, the listener briefly has to figure out that the subject has changed.

Using а я сажусь… clearly marks the contrast and the new subject. In normal speech and writing, including “я” is preferred here.