В детской комнате лежали кукла, кубики, пазл, пирамидка, медвежонок и красная машинка.

Because after в meaning in, Russian usually uses the prepositional case for location.

• детская комната = children’s room / playroom in the dictionary form
• в детской комнате = in the children’s room

Both words change because they are part of the same phrase:

• детская → детской
• комната → комнате

This is very common in Russian:

• в доме = in the house
• в школе = in the school
• в большой комнате = in the big room

So в детской комнате is the normal way to say in the children’s room.

Yes. The adjective детский changes to match the noun it describes.

The noun комната is:

• feminine
• singular

So the adjective must also be feminine singular:

• masculine: детский
• feminine: детская
• neuter: детское
• plural: детские

Then, because the whole phrase is in the prepositional case after в, both words change again:

• nominative: детская комната
• prepositional: в детской комнате

So детской is feminine because it agrees with комнате.

Because the subject is really the whole list together, not just one item.

The sentence lists several things:

• кукла
• кубики
• пазл
• пирамидка
• медвежонок
• красная машинка

Together, they form a plural subject, so the verb is plural:

• singular: лежала кукла = a doll was lying there
• plural: лежали кукла, кубики, пазл... = a doll, blocks, a puzzle... were lying there

This is normal in Russian. Even if some listed nouns are singular, the whole group is plural, so лежали is correct.

Russian word order is more flexible than English word order.

In this sentence:

• В детской комнате лежали ...

the structure sounds natural because it introduces the location first, then says what was there.

This is very common when describing a scene:

• На столе лежала книга. = There was a book on the table.
• В комнате стоял шкаф. = There was a wardrobe in the room.
• На полу лежали игрушки. = There were toys on the floor.

So лежали before the list is not strange. It often has an existential/descriptive feel, like there were ... in the room.

Russian often uses more specific position verbs where English might just say there was / there were.

• были = were
• лежали = were lying / lay

Here the toys are imagined as being lying somewhere, probably on the floor or around the room. So лежали sounds more vivid and natural than just были.

Russian often distinguishes between:

• стоять = stand
• сидеть = sit
• лежать = lie

For example:

• На столе стояла ваза. = A vase was standing on the table.
• На стуле сидела кошка. = A cat was sitting on the chair.
• На кровати лежала книга. = A book was lying on the bed.

So лежали helps describe the physical position of the toys.

Because the speaker is referring to more than one block.

• кубик = one block / cube
• кубики = blocks

The other items are presented as single objects:

• кукла = one doll
• пазл = one puzzle
• пирамидка = one stacking toy / pyramid toy
• медвежонок = one teddy bear / bear cub toy
• машинка = one toy car

So the list mixes singular and plural nouns naturally, just as English can:

a doll, blocks, a puzzle, a pyramid toy, a teddy bear, and a red toy car

Because adjectives must agree with the noun they describe.

машинка is:

• feminine
• singular
• nominative here

So red must also be feminine singular nominative:

• masculine: красный
• feminine: красная
• neuter: красное
• plural: красные

That gives:

• красная машинка = red toy car / little car

The adjective and noun match in gender, number, and case.

Машинка is a diminutive form of машина.

• машина usually means car or machine
• машинка can mean little car, toy car, or sometimes just a more affectionate/smaller form

In a sentence about toys in a children’s room, машинка strongly suggests a toy car, not a real car.

Russian uses diminutives a lot, especially with:

• children
• toys
• affectionate speech
• small objects

Other examples:

• кот → котик
• дом → домик
• кукла → куколка

So красная машинка sounds very natural in a toy context.

Literally, медвежонок means bear cub.

But in the context of toys in a child’s room, it very often means a teddy bear or little bear toy.

It comes from:

• медведь = bear
• медвежонок = bear cub / little bear

So in this sentence, a learner should understand it as the toy, not a real animal.

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

So:

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

The exact meaning depends on context.

In this sentence, English would naturally translate the items with a or sometimes without an article in a list, but Russian simply does not mark that distinction with separate words.

This is something English speakers have to get used to: Russian usually leaves definiteness to context.

Yes, пазл is a singular masculine noun in Russian.

It is a borrowing from English puzzle, but in Russian it behaves like a normal noun:

• singular: пазл
• plural: пазлы

Because it ends in a consonant, learners can usually expect it to be masculine.

So here:

• пазл = one puzzle

And if there were several, you might see:

• пазлы лежали на столе = puzzles were lying on the table

Because this is a standard list in Russian punctuation, just like in English.

Items in a list are separated by commas:

• кукла, кубики, пазл, пирамидка, медвежонок

But before the final и (and), Russian normally does not use a comma:

• медвежонок и красная машинка

So the punctuation follows the normal pattern for a simple series.

It would not be grammatically wrong, but it would sound less vivid.

• были = there were / were
• лежали = were lying

Using были simply states existence:

• В детской комнате были игрушки. = There were toys in the children’s room.

Using лежали paints a clearer picture:

• the toys were lying there, probably scattered around

So лежали is the more natural descriptive choice in this context.