Дочка так замечталась у окна, что мгновенно забыла про домашнее задание.

Both mean daughter, but they are different in tone.

• дочь is the neutral, standard word.
• дочка is a more affectionate, everyday form, like daughter / little daughter / dear daughter depending on context.

In Russian, diminutive forms are very common and do not always mean literal smallness. Here дочка sounds warm and natural in a narrative sentence.

Because the subject, дочка, is feminine singular, and both verbs are in the past tense.

In Russian past tense, verbs agree in gender and number:

• masculine: usually no extra ending, e.g. забыл
• feminine: -а, e.g. забыла
• neuter: -о
• plural: -и

So:

• дочка замечталась
• дочка забыла

both use feminine past forms.

Замечтаться is a reflexive verb meaning something like:

• to get lost in thought
• to drift into daydreaming
• to become absorbed in one's dreams/thoughts

The -ся ending often makes a verb feel inward, state-like, or centered on the subject’s own experience.

Here, замечталась does not mean she deliberately dreamed something. It means she slipped into a dreamy state.

A useful comparison:

• мечтать = to dream, to daydream
• замечтаться = to start daydreaming so much that you become absorbed in it

So the reflexive form gives it that sense of becoming carried away by one's thoughts.

This is a very common Russian pattern meaning:

• so ... that ...

So:

• так замечталась, что... = she got so lost in daydreaming that...

This structure introduces a result:

• first part: the degree or intensity
• second part: what happened as a consequence

Other examples:

• Он так устал, что сразу уснул.
  He was so tired that he fell asleep immediately.

• Она так спешила, что забыла телефон.
  She was in such a hurry that she forgot her phone.

Замечтаться is perfective.

That matters because perfective verbs in Russian often present an action as:

• a completed event
• a change of state
• a single whole occurrence

Here замечталась suggests she fell into a dreamy state, not that she was simply daydreaming in a general ongoing way.

Compare:

• мечтала у окна = she was daydreaming by the window
• замечталась у окна = she got carried away daydreaming by the window

The perfective form fits well because it helps create the chain of events:

1. she got lost in daydreaming
2. as a result, she forgot her homework

У окна means by the window or near the window.

After у, Russian normally uses the genitive case, so:

• окно → окна

Why not на окне?

• у окна = by / near the window
• на окне = on the windowsill / on the window surface

So дочка замечталась у окна means she was standing or sitting by the window, not literally on it.

These mean slightly different things.

• забыла домашнее задание can mean she forgot the homework itself — for example, left it somewhere, forgot to bring it, or forgot what it was.
• забыла про домашнее задание means she forgot about the homework.

So про makes it clear that the meaning is forgot about, not necessarily forgot the item/task itself.

This is very natural in spoken and everyday Russian.

After про, Russian uses the accusative case.

The noun phrase is:

• dictionary form: домашнее задание
• accusative singular: домашнее задание

Because задание is neuter and inanimate, the nominative and accusative forms are the same.

The adjective also matches:

• домашнее = neuter singular accusative, matching задание

So:

• про домашнее задание = about the homework

Yes, but the style changes.

• забыть про что-то = very common, conversational, natural
• забыть о чём-то = also correct, often a bit more neutral or bookish

So:

• забыла про домашнее задание sounds very natural in everyday speech
• забыла о домашнем задании is also correct, just slightly different in tone

Both mean forgot about the homework.

Мгновенно is an adverb meaning:

• instantly
• in a flash
• immediately

It emphasizes how quickly she forgot.

So the sentence is not just saying she forgot, but that the forgetting happened right away as a result of becoming absorbed in daydreaming.

No, Russian word order is flexible, but the given order is very natural.

Current version:

• Дочка так замечталась у окна, что мгновенно забыла про домашнее задание.

This order sounds smooth because it presents:

1. the subject: дочка
2. the cause: так замечталась у окна
3. the result: что мгновенно забыла...

You could move things around for emphasis, for example:

• У окна дочка так замечталась, что мгновенно забыла про домашнее задание.
• Дочка у окна так замечталась, что мгновенно забыла про домашнее задание.

These are still grammatical, but the original is the most neutral and natural.

Not quite. A simple to dream is often too weak or too general.

Замечтаться usually implies:

• drifting into daydreaming
• becoming absent-minded because of your thoughts
• getting carried away by dreamy thinking

So in this sentence, a better sense is:

• she got lost in daydreaming
• she became so absorbed in her thoughts
• she drifted off into a dreamlike state

That nuance helps explain why she then forgot about her homework.