Будучи ребёнком, она жила у моря.

Будучи is an adverbial participle from быть. In this sentence it means something like being, while being, or when she was.

So:

• Будучи ребёнком = being a child / when she was a child

This form does not change for gender or number, and it gives background information for the main action.

An important rule: the subject of будучи must be the same as the subject of the main verb. Here both refer to она.

Because after будучи, Russian normally uses the instrumental case to describe a role, identity, or state.

So:

• ребёнок = nominative
• ребёнком = instrumental

This is very common in patterns like:

• будучи студентом = being a student
• будучи врачом = being a doctor
• будучи ребёнком = being a child

So ребёнком is correct because it means in the state/role of a child.

It is correct, but it sounds a bit more written, formal, or bookish than the most everyday version.

In casual speech, Russians would often say:

• Когда она была ребёнком, она жила у моря.
• В детстве она жила у моря.

So будучи + instrumental is perfectly good Russian, but it is often more common in writing, narration, or more polished speech.

Because будучи ребёнком is an adverbial participle phrase, and these phrases are normally separated by commas in Russian.

So the structure is:

• Будучи ребёнком, = background circumstance
• она жила у моря = main statement

The comma works similarly to English punctuation in phrases like:

• When she was a child, ...
• Being a child, ...

Жила is the past tense, feminine singular form of жить.

It is feminine because the subject is она.

Compare:

• он жил = he lived
• она жила = she lived
• они жили = they lived

So жила agrees with она.

Also, жить is imperfective, which fits well here because living somewhere is usually seen as an ongoing state, not a single completed action.

Моря is the genitive singular form of море.

That is because у requires the genitive case.

So:

• море = sea
• у моря = by the sea / near the sea

This is a very common pattern:

• у дома = by the house
• у окна = by the window
• у реки = by the river
• у моря = by the sea

This is a very common learner question.

• у моря means by the sea, near the sea, emphasizing location next to it
• на море often means at the seaside or to the seaside, especially in the context of travel or vacation

Examples:

• Она жила у моря. = She lived by the sea.
• Летом они ездили на море. = In summer they went to the seaside.

So in this sentence, у моря is the natural choice because it describes where she lived.

Yes, very often.

Russian frequently leaves out subject pronouns when the meaning is clear from context. So in the right context, you could say:

• Будучи ребёнком, жила у моря.

However, keeping она makes the sentence clearer, especially because будучи must refer to the same subject as the main verb. So including она is completely natural and helps the reader.

Yes. Russian word order is flexible.

These are all possible:

• Будучи ребёнком, она жила у моря.
• Она, будучи ребёнком, жила у моря.
• Она жила у моря, будучи ребёнком.

The original version is very natural. It presents the background first, then the main fact.

Changing the order can slightly change emphasis, but not the basic meaning.

Yes: the biggest thing is the same-subject rule.

In a sentence with будучи, the person described by будучи must be the same person who does the main action.

Here that works perfectly:

• Будучи ребёнком, она жила у моря.
  • she was the child
  • she lived by the sea

That is why this sentence is natural and grammatically correct.

Also, remember that будучи is correct but somewhat more formal than simpler alternatives like когда она была ребёнком or в детстве.