Один ребёнок сидит на скамейке, а другой играет на качелях.

In this sentence, один ... другой works as a pair meaning one ... the other.

So:

• Один ребёнок = one child
• другой = the other (child)

Here один is not mainly being used to count. It helps set up a contrast between two people:

• Один ребёнок сидит... = One child is sitting...
• а другой играет... = and the other is playing...

This is a very common Russian pattern.

Russian often leaves out a noun if it is already clear from context.

So after Один ребёнок..., the word другой naturally means the other child, and repeating ребёнок would be unnecessary.

Both are possible:

• Один ребёнок сидит на скамейке, а другой играет на качелях.
• Один ребёнок сидит на скамейке, а другой ребёнок играет на качелях.

The version without the repeated noun sounds more natural and less repetitive.

А is often used to connect two clauses that are being compared or contrasted.

In this sentence, it is not a strong contradiction like but, and it is not just a simple and either. It is more like:

• while
• whereas
• and meanwhile
• sometimes simply and, but with contrast

So:

• Один ребёнок сидит на скамейке, а другой играет на качелях.

means something like:

• One child is sitting on the bench, while the other is playing on the swings.

Comparison of the conjunctions:

• и = simple and
• но = stronger but
• а = contrast, comparison, change of topic

The correct spelling is ребёнок.

The letter ё shows both:

• the sound yo
• the stress

So ребёнок is pronounced roughly ree-BYO-nak.

In many printed texts, Russians often write е instead of ё, so you may also see ребенок. But the meaning and pronunciation are the same.

For learners, it is very helpful to remember the real form with ё:

• ребёнок
• plural: дети
• genitive plural: детей

Both are 3rd person singular present tense forms.

• сидеть = to sit
• сидит = he/she is sitting
• играть = to play
• играет = he/she is playing

They match the subject:

• ребёнок = child → singular
• другой = the other one → also singular

So Russian uses singular verb forms:

• ребёнок сидит
• другой играет

Russian present tense often covers both simple present and present continuous in English, depending on context:

• сидит = sits / is sitting
• играет = plays / is playing

Here the natural English translation is continuous: is sitting, is playing.

Because after на when talking about location, Russian usually uses the prepositional case.

The basic noun is:

• скамейка = bench

But after на meaning on or at, it becomes:

• на скамейке = on the bench

This is the prepositional singular form of a feminine noun ending in -а:

• скамейка → скамейке

So:

• сидит на скамейке = is sitting on the bench

На качелях is also after на in the sense of location, so it is in the prepositional case.

The noun is:

• качели = swings

But качели is a special noun because it is normally used only in the plural.

Its prepositional plural form is:

• на качелях = on the swings

So the pattern is:

• качели → на качелях

This is why the ending is different from скамейке:

• скамейка is singular feminine
• качели is plural

Grammatically, качели is a plural-only noun.

That means Russian normally treats it as plural, even when English might say a swing or the swings depending on the situation.

Examples:

• качели стоят во дворе = the swings are in the yard
• дети играют на качелях = the children are playing on the swings

A native speaker will usually use plural forms with this word.

Yes, both are possible, but they emphasize slightly different things.

• играет на качелях = is playing on the swings
• качается на качелях = is swinging / is rocking on the swings

So:

• играет focuses on the child’s activity as play
• качается focuses more specifically on the motion of swinging

In your sentence, играет на качелях is completely natural if the idea is that the child is playing on the playground equipment.

Here другой is in the nominative masculine singular.

Why?

Because it stands for другой ребёнок = the other child.

• ребёнок is masculine
• so the adjective/pronoun другой must match it

That is why it is:

• другой, not другая, другое, or другие

Compare:

• другой ребёнок = the other child
• другая девочка = the other girl
• другое место = the other place
• другие дети = the other children

Because а joins two separate clauses, and Russian normally puts a comma before it.

Here the two clauses are:

1. Один ребёнок сидит на скамейке
2. а другой играет на качелях

Each clause has its own subject and verb, so the comma is required.

This is very standard Russian punctuation.

Yes. Russian word order is more flexible than English, because cases show grammatical relationships.

The neutral, natural order here is:

• Один ребёнок сидит на скамейке, а другой играет на качелях.

But other orders are possible for emphasis, for example:

• На скамейке сидит один ребёнок, а на качелях играет другой.

This version emphasizes the locations more strongly.

Even though word order can change, the original sentence is the most straightforward for a learner.

A helpful stress guide is:

• оди́н ребёнок сиди́т на скаме́йке, а друго́й игра́ет на каче́лях

Approximate pronunciation:

• a-DEEN ree-BYO-nak see-DEET na ska-MYEY-ke, a droo-GOY ig-RA-yet na ka-CHE-lyakh

A few useful notes:

• ё in ребёнок is always stressed
• скамейке has stress on -мей-
• качелях has stress on че

Yes. Скамейка is the normal everyday word for bench.

So:

• на скамейке = on the bench

You may also come across related words like:

• лавочка = bench, often a small bench; can sound a bit more colloquial
• скамья = a different but related word, often more formal or specific

For a basic learner sentence, скамейка is the standard choice.

Russian has no articles like a or the.

Instead, meaning comes from context and from words like:

• один = one
• другой = other / the other

So:

• Один ребёнок naturally means one child
• другой naturally means the other one

Russian speakers do not need articles here, because the contrast between the two children is already clear.