Малышка посадила куклу рядом с медвежонком и начала собирать пазл на ковре.

Yes, посадить can mean to plant, but it also very commonly means to seat, to sit someone down, or to place something in a sitting position.

So in:

Малышка посадила куклу...

it means the little girl sat the doll down or placed the doll.

This is a very natural verb with dolls, toys, children, and people:

• посадить ребёнка на стул = to sit a child on a chair
• посадить куклу на диван = to sit a doll on the sofa

So the exact meaning depends on the object and the context.

Because куклу is the accusative singular form of кукла.

The noun кукла is the direct object of посадила:

• Малышка посадила кого? что? → куклу

For feminine nouns ending in -а, the accusative singular usually changes -а to -у:

• кукла → куклу
• девочка → девочку
• книга → книгу

So куклу is there because the doll is the thing being acted on.

The expression is рядом с = next to / beside.

It is a fixed pattern:

• рядом с кем? с чем?

So you need the preposition с after рядом.

Examples:

• рядом с домом = next to the house
• рядом с мамой = next to mom
• рядом с медвежонком = next to the teddy bear / bear cub

You should learn рядом с as a chunk.

Because с in the expression рядом с requires the instrumental case.

The base noun is медвежонок. In the instrumental singular, it becomes:

• медвежонок → медвежонком

So:

• рядом с медвежонком = next to the teddy bear / bear cub

This is the answer to:

• с кем? с чем? → с медвежонком

Literally, yes: медвежонок means bear cub.

But in everyday speech it is also very commonly used for a teddy bear, especially in child-related contexts.

In this sentence, since we have a little girl, a doll, and a puzzle on the floor, медвежонок is most naturally understood as a teddy bear.

So the phrase suggests a toy scene:

• she sat the doll next to the teddy bear

After начать / начинать (to begin / to start), Russian normally uses an imperfective infinitive.

So:

• начала собирать = began to assemble / started assembling

This is the usual pattern:

• начал читать = started reading
• начала писать = started writing
• начали играть = started playing

Even though начать itself is perfective here (начала = she began), the second verb is usually imperfective because it describes the process that was started.

So начала собрать would generally sound wrong in standard usage.

This sentence shows a very common aspect pattern.

1. посадила

This is perfective past from посадить.
It presents the action as completed:

• she sat the doll down

2. начала

This is also perfective past from начать.
It marks the beginning of an action as a completed event:

• she began

3. собирать

This is imperfective infinitive.
It focuses on the activity/process:

• to assemble / to put together

So the sentence structure is roughly:

• She completed one action
• and then began another action/process

That is very natural Russian narration.

Here пазл is the direct object of собирать, so it is in the accusative case.

But пазл is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular:

• пазл → пазл

So:

• собирать пазл = to assemble a puzzle

Compare:

• вижу стол = I see a table
• собираю пазл = I’m assembling a puzzle

No visible ending change is needed here.

Yes, it is normal and understandable.

собирать пазл means to assemble a puzzle / do a jigsaw puzzle.

You may also hear:

• собирать головоломку
• складывать пазл

But собирать пазл is common and natural in modern Russian.

Because на ковре answers the question where?, not onto what?

Russian uses different cases after на depending on meaning:

Location: where?

Use the prepositional case:

• на ковре = on the carpet/rug

Direction: onto where?

Use the accusative case:

• на ковёр = onto the carpet/rug

So here the puzzle is being assembled on the rug, meaning location, not movement.

It is the prepositional singular of ковёр.

• ковёр = rug, carpet
• на ковре = on the rug

This follows the pattern:

• на столе = on the table
• в доме = in the house
• на ковре = on the rug

One small thing to notice: the vowel and stress shift from ковёр to ковре. That is just part of how this noun declines.

Малышка is more affectionate and more specific than plain girl.

It comes from маленький / малыш and means something like:

• little girl
• little one
• tiny girl
• sometimes even baby girl, depending on context

It has a warm, tender, child-centered feel.

Compare:

• девочка = girl
• малышка = little girl / little one

So Малышка посадила куклу... sounds sweet and story-like.

Yes, both have a diminutive or affectionate feel.

малышка

A tender word for a small child, especially a girl.

медвежонок

Literally little bear / bear cub, and often used affectionately for a teddy bear.

This gives the whole sentence a soft, childlike tone. It sounds like a scene from a children’s story or a parent describing a child at play.

Yes, Russian word order is flexible, but this order is very natural.

Малышка посадила куклу рядом с медвежонком и начала собирать пазл на ковре.

This order presents events clearly:

1. subject: Малышка
2. first action: посадила куклу рядом с медвежонком
3. second action: начала собирать пазл на ковре

A different word order is possible for emphasis, for example:

• На ковре малышка начала собирать пазл...
• Куклу малышка посадила рядом с медвежонком...

But the original is the most neutral and natural storytelling order.

Because Russian has no articles.

So:

• кукла can mean a doll or the doll
• пазл can mean a puzzle or the puzzle
• ковёр can mean a rug or the rug

The exact meaning depends on context.

In this sentence, English would probably use:

• the doll
• the teddy bear
• a puzzle or the puzzle
• on the rug

But Russian leaves that to context instead of using articles.

A natural stress pattern would be:

Малы́шка посади́ла ку́клу ря́дом с медвежо́нком и начала́ собира́ть па́зл на ковре́.

Useful stress points:

• малы́шка
• посади́ла
• ку́клу
• ря́дом
• медвежо́нком
• начала́
• собира́ть
• па́зл
• ковре́

Stress is important in Russian, especially for words like:

• начала́
• ковре́

Yes, that is a very good natural translation.

A few slightly different English versions are also possible:

• The little girl sat the doll next to the teddy bear and began assembling a puzzle on the rug.
• The little girl placed the doll beside the teddy bear and started putting together a puzzle on the carpet.

The Russian itself is neutral and natural enough to allow several good English translations.