Дома сын открыл конструктор и сразу начал собирать мост.

Yes, дом is house/home as a noun. But дома can also be a fixed adverb meaning at home.

So in this sentence:

• Дома = at home
• it is not the plural of дом here

This is very common in Russian:

• Я дома. = I’m at home.
• Мы остались дома. = We stayed at home.

So Дома сын... means something like At home, the son...

Russian word order is flexible, so putting Дома first helps set the scene right away: At home...

The sentence could also be written as:

• Сын дома открыл конструктор...

but Дома сын... sounds natural if the speaker wants to start with the location.

In Russian, the beginning of the sentence often contains what the speaker wants to frame first: place, time, or topic.

Сын is the nominative singular form, used for the subject of the sentence.

Here, сын is the person doing the actions:

• открыл = opened
• начал собирать = started assembling

So he is the subject, and Russian uses the nominative for the subject.

In this context, конструктор usually means a construction set/building set, especially a toy with pieces that can be assembled into something.

So it does not mean designer here.

Russian конструктор can mean different things depending on context:

• designer / engineer-constructor in technical contexts
• construction set in everyday or child-related contexts

Since the sentence talks about assembling a bridge, the meaning is clearly construction set.

Открыл is the past tense masculine singular form of открыть.

In Russian past tense:

• masculine: usually ends in a consonant → открыл
• feminine: открыла
• neuter: открыло
• plural: открыли

Because the subject is сын (a masculine noun), the verb is masculine too.

So сын открыл = the son opened.

This is a very common Russian pattern:

• начать
  • infinitive = to start doing something

So:

• начал = started
• собирать = to assemble / to build

Together:

• начал собирать = started assembling

Russian often uses this structure just like English uses started + -ing or started to + verb.

This is about verb aspect, which is very important in Russian.

• собирать = imperfective
• собрать = perfective

After начать, Russian often uses the imperfective infinitive when focusing on the beginning of a process:

• начал собирать мост = started assembling a bridge

This emphasizes that the action began as an activity/process.

If you said начал собрать, that would sound wrong in standard Russian.

Good question. Мост is the direct object of собирать, so it is in the accusative case.

But for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: мост
• accusative: мост

That is why it looks unchanged.

Compare:

• Я вижу стол. = I see a table.
• Я строю мост. = I’m building a bridge.

But with animate masculine nouns, accusative often matches the genitive instead.

Сразу means right away / immediately.

It shows that after opening the construction set, he did not wait—he began assembling the bridge at once.

So it adds a sense of quick sequence:

1. he opened the construction set
2. he immediately started assembling the bridge

Russian has no articles like English a/an/the.

So nouns such as сын, конструктор, and мост do not automatically include those words. English has to choose them based on context.

For example:

• сын could be the son or sometimes just son
• конструктор could be the construction set or a construction set
• мост could be a bridge or the bridge

The exact English choice depends on the broader context, not on a separate Russian word.

Yes, Russian word order can often be changed without changing the basic meaning, because the grammatical roles are shown by case and verb forms more than by position.

For example, these are all possible:

• Дома сын открыл конструктор и сразу начал собирать мост.
• Сын дома открыл конструктор и сразу начал собирать мост.
• Конструктор сын открыл дома и сразу начал собирать мост.

The core meaning stays similar, but the focus changes.

• Дома first = setting/place is highlighted
• Сын first = the son is highlighted
• Конструктор first = the construction set is highlighted

So Russian word order is flexible, but not random: it often reflects emphasis and information flow.