Дети хотели лепить снеговика, хотя снег был слишком мокрым.

Because the subject is дети = children, which is plural. In Russian, past-tense verbs agree with the subject in number and, in the singular, also gender.

So:

• он хотел = he wanted
• она хотела = she wanted
• оно хотело = it wanted
• они хотели = they wanted

Here, дети хотели literally means the children wanted.

After хотеть = to want, Russian normally uses an infinitive to say what someone wanted to do.

So:

• хотели лепить = wanted to make/model/sculpt
• literally: wanted to sculpt a snowman

This works much like English wanted to do something.

This is an aspect question.

• лепить is imperfective
• слепить is perfective

In this sentence, лепить suggests the activity itself: they wanted to make / be making a snowman. The focus is on the process or intention, not on successfully finishing it.

If you said хотели слепить снеговика, that would sound more like they wanted to make a snowman completely / finish making one.

Both can be possible in different contexts, but лепить fits well when the sentence is about trying or wanting to do the activity, especially since the snow was too wet.

Because it is the direct object of лепить, so it goes into the accusative case.

The dictionary form is:

• снеговик = snowman

But in the sentence:

• лепить снеговика = to make a snowman

Since снеговик is a masculine animate noun, its accusative singular looks like the genitive singular:

• nominative: снеговик
• accusative: снеговика

This is a very important pattern in Russian:

• вижу брата = I see my brother
• вижу снеговика = I see a snowman

But with an inanimate masculine noun:

• вижу стол = I see a table

Дети is an irregular plural meaning children.

The usual singular is:

• ребёнок = child

So the pair is:

• ребёнок = child
• дети = children

This is not a regular plural pattern, so it is something learners usually just memorize.

Because хотя means although / even though and introduces a subordinate clause.

The sentence has two parts:

• Дети хотели лепить снеговика
• хотя снег был слишком мокрым

In Russian, subordinate clauses introduced by words like хотя, потому что, если, когда are usually separated by a comma.

So the comma here is required.

Because снег is a masculine singular noun.

In the past tense, быть agrees with the subject:

• был = masculine singular
• была = feminine singular
• было = neuter singular
• были = plural

Since снег is masculine, Russian uses был:

• снег был = the snow was

In снег был слишком мокрым, the word мокрым is a predicate adjective after был. Russian often uses the instrumental case for predicate nouns and adjectives, especially in the past and future, or when describing a state or condition.

So:

• мокрый = wet
• мокрым = wet (instrumental form)

This is why the sentence says:

• снег был слишком мокрым = the snow was too wet

You may also hear nominative in some contexts, such as снег был слишком мокрый, especially in colloquial speech, but мокрым is very natural and standard here.

Слишком means too, in the sense of more than is good or possible.

• слишком мокрый = too wet
• очень мокрый = very wet

That difference matters here:

• снег был слишком мокрым means the snow was so wet that it caused a problem for making a snowman.
• If you said очень мокрым, it would only mean very wet, without necessarily implying that it was unsuitable.

Yes. Russian word order is fairly flexible because case endings show grammatical roles.

This sentence could also be:

• Хотя снег был слишком мокрым, дети хотели лепить снеговика.

That means the same thing: Although the snow was too wet, the children wanted to make a snowman.

The difference is mostly about focus and flow:

• Дети хотели... хотя... starts with the children and their desire
• Хотя снег был... starts with the obstacle or contrast

Russian has no articles, so there is no separate word for a or the.

So снеговика can mean:

• a snowman
• the snowman

The exact meaning depends on context. In this sentence, English would usually translate it as a snowman, but Russian itself does not mark that distinction with an article.