Дети любят мастерить что‑то из картона и цветной бумаги.

Дети is in the nominative plural because it is the subject of the sentence: the children are the ones doing the liking.

The basic singular/plural pattern is:

• ребёнок = child
• дети = children

Russian often uses дети as the normal plural form for ребёнок, even though it looks irregular from an English learner’s point of view.

In this sentence:

• Дети = Children
• любят = like

So Дети любят... means Children like...

Because the subject Дети is plural.

Russian verbs in the present tense agree with the subject in person and number:

• я люблю = I like
• ты любишь = you like
• он/она любит = he/she likes
• они любят = they like

Since дети means they in terms of grammar, the verb must be любят.

So:

• Дети любят = The children like

Мастерить means something like to make, to craft, to build with your hands, especially simple handmade things.

It is more specific than делать.

• делать = to do / to make (very general)
• мастерить = to craft, tinker, make things by hand

So in this sentence, мастерить suggests that the children like making little handmade objects, probably arts and crafts.

That is why мастерить sounds more natural here than the broader делать.

This is a very common Russian pattern.

The first verb, любят, is a conjugated verb meaning like.
The second verb, мастерить, stays in the infinitive because it expresses the action that they like doing.

So:

• любят мастерить = like to make / like making

This is similar to English:

• Children like making...
• Children like to make...

Russian often uses:

• любить + infinitive

Examples:

• Я люблю читать. = I like reading.
• Они любят гулять. = They like walking.
• Дети любят мастерить. = Children like crafting.

Что‑то means something.

It is an indefinite pronoun:

• что = what
• что‑то = something

In this sentence, it means the children like making some kind of thing, without saying exactly what.

So:

• мастерить что‑то = to make something

The hyphen is important. Russian writes this form with a hyphen:

• что‑то
• кто‑то = someone
• где‑то = somewhere

Yes, it could.

You can say:

• Дети любят мастерить из картона и цветной бумаги.

That would still mean that children like making things out of cardboard and colored paper.

Adding что‑то makes the idea slightly more explicit: they like making something out of those materials.

So:

• without что‑то = more general
• with что‑то = explicitly says something

Both are natural.

Because the preposition из requires the genitive case.

Из often means:

• from
• out of
• made of

After из, the noun usually goes into the genitive.

So:

• картон → картона
• цветная бумага → цветной бумаги

That is why the sentence says:

• из картона
• из цветной бумаги

not:

• из картон
• из цветная бумага

Because the whole phrase is in the genitive case after из.

The base form is:

• цветная бумага = colored paper

But after из, both the adjective and noun must change:

• цветная бумага → цветной бумаги

Why?

• бумага is feminine singular
• after из, it becomes genitive singular
• the adjective must agree with the noun

So:

• nominative: цветная бумага
• genitive: цветной бумаги

This is a standard adjective-noun agreement pattern in Russian.

In Russian, бумага here is treated as a material noun, like paper in English.

So цветной бумаги means colored paper as a material, not colored papers as separate sheets.

Likewise:

• из дерева = out of wood
• из металла = out of metal
• из бумаги = out of paper

Russian commonly uses the singular for materials in this kind of phrase.

Because one preposition can govern both nouns when they are joined by и.

So:

• из картона и цветной бумаги

means:

• out of cardboard and colored paper

Russian does not need to repeat из before the second noun if the meaning is clear.

You could repeat it:

• из картона и из цветной бумаги

but that is less natural here unless you want extra emphasis or clearer separation.

Yes, мастерить is imperfective.

That makes sense because the sentence describes a general liking or habitual activity, not one completed result.

• мастерить = to craft / to make by hand (process, repeated activity)
• смастерить = to make/craft successfully, complete one item

Here the idea is:

• children like doing crafts
• children like making things

So the imperfective мастерить is the natural choice.

Russian usually uses the imperfective after любить when talking about activities someone likes in general.

The given word order is very natural:

• Дети любят мастерить что‑то из картона и цветной бумаги.

It presents the information in a neutral way:

1. Дети — who
2. любят — what they feel
3. мастерить — what they like doing
4. что‑то из картона и цветной бумаги — what they make and from what

Russian word order is more flexible than English, so other versions are possible, for example:

• Дети любят что‑то мастерить из картона и цветной бумаги.

This is also natural and puts что‑то closer to мастерить.

But the original sentence is perfectly standard and easy to understand.