Перед сном я дочитаю рассказ.

Because перед in the meaning before takes the instrumental case.

• сон = sleep
• instrumental singular: сном

So:

• перед сном = before sleep / before going to bed

This is a very common pattern in Russian:

• перед уроком = before class
• перед обедом = before lunch
• перед сном = before sleep

Literally, it means before sleep. In natural English, it often corresponds to:

• before going to sleep
• before bed
• at bedtime (depending on context)

So in this sentence, перед сном suggests the time just before the speaker goes to sleep, not necessarily some abstract idea of sleep.

Дочитаю is a perfective verb, and perfective verbs in Russian can express a completed future action.

Here the speaker means not just I will be reading, but I will finish reading the story before going to sleep.

Compare:

• я читаю = I am reading / I read
• я буду читать = I will be reading
• я прочитаю = I will read / I will read through
• я дочитаю = I will finish reading

So дочитаю is used because the focus is on reaching the end of the story.

The prefix до- often adds the idea of reaching an endpoint, finishing, or doing something to the end.

So:

• читать = to read
• дочитать = to finish reading

In this sentence, дочитаю рассказ implies that the speaker has probably already started the story and plans to finish it before sleep.

This is an important nuance: дочитать often suggests finishing something already in progress.

Both can involve reading a story completely, but the nuance is different.

• прочитаю рассказ = I will read the story / I will read it through
• дочитаю рассказ = I will finish reading the story

So дочитаю often implies:

• the reading may already have begun
• the important point is finishing what remains

If someone is partway through a story and says Перед сном я дочитаю рассказ, it strongly suggests I’ll finish the rest of the story before bed.

Рассказ is the direct object, so it is in the accusative case.

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

So:

• nominative: рассказ
• accusative: рассказ

That is why you do not see a different ending here.

Compare with an animate masculine noun:

• я вижу брата = I see my brother

There, the accusative is different.

Formally, it looks like a present-tense personal ending, but because дочитать is perfective, this form has future meaning.

So:

• imperfective present forms = present time
• perfective “present” forms = future time

Therefore:

• я дочитаю = I will finish reading

Russian does not normally use perfective verbs for true present-time actions.

Yes, Russian word order is flexible.

This version puts the time expression first:

• Перед сном я дочитаю рассказ.

That feels natural if the speaker wants to set the time frame first: Before bed, I’ll finish the story.

Other possible orders include:

• Я дочитаю рассказ перед сном.
• Рассказ я дочитаю перед сном.

These are all grammatical, but the emphasis shifts slightly.

• Перед сном... emphasizes when
• Я дочитаю рассказ... is more neutral
• Рассказ я дочитаю... emphasizes the story

No. Russian has no articles like a or the.

So рассказ can mean:

• a story
• the story

The exact meaning depends on context.

In this sentence, English might translate it as the story if both speaker and listener already know which story is meant, or a story if it is being mentioned more generally. Russian leaves that unstated unless the context makes it clear.

Grammatically, до сна is possible in some contexts, but перед сном is much more natural here.

• перед сном = before going to sleep / before bed
• до сна sounds more literal or less idiomatic in everyday speech for this situation

So if you want the natural everyday phrase before bed, перед сном is the best choice.

Дочитаю is:

• 1st person
• singular

So it means I will finish reading.

A quick mini-paradigm:

• я дочитаю = I will finish reading
• ты дочитаешь = you will finish reading
• он / она дочитает = he / she will finish reading
• мы дочитаем = we will finish reading
• вы дочитаете = you will finish reading
• они дочитают = they will finish reading