Перед сном я хочу дочитать книгу.

Перед сном means before sleep or more naturally before going to bed / before falling asleep.

In everyday English, you would usually translate it as something like:

• Before bed, I want to finish reading the book.
• Before going to sleep, I want to finish reading the book.

So сон is literally sleep, but in this phrase it often refers to the time right before sleeping.

Because the preposition перед normally requires the instrumental case when it means before.

So:

• nominative: сон = sleep
• instrumental: сном

That is why you get:

• перед сном = before sleep

This is a very common pattern:

• перед ужином = before dinner
• перед работой = before work
• перед экзаменом = before the exam

Not exactly, though they can be similar in some contexts.

• перед сном is the most natural expression for before bed / before going to sleep
• до сна is more literally before sleep, but it sounds less idiomatic in many everyday situations

So if you want a natural everyday phrase, перед сном is the better choice here.

Because дочитать means to finish reading or to read to the end.

Compare:

• читать = to read / to be reading
• дочитать = to finish reading

The prefix до- often adds the idea of reaching the end or completing something.

So:

• Я хочу читать книгу. = I want to read a book / be reading a book.
• Я хочу дочитать книгу. = I want to finish reading the book.

Here, до- adds the sense of completion: doing something all the way to the end.

So дочитать книгу means not just reading it, but finishing it.

This same idea appears in other verbs too:

• писать = to write

• дописать = to finish writing

• смотреть = to watch
• досмотреть = to finish watching

So до- often suggests finishing what was started.

Because Russian uses хотеть + infinitive just like English uses to want + infinitive.

So:

• я хочу дочитать = I want to finish reading

This is a very common structure:

• Я хочу спать. = I want to sleep.
• Я хочу поехать домой. = I want to go home.
• Я хочу купить книгу. = I want to buy a book.

Because книга is the direct object of дочитать.

You are finishing reading what?
→ книгу

So the noun goes into the accusative case:

• nominative: книга
• accusative: книгу

This is normal for a direct object with a transitive verb:

• читать книгу = to read a book
• купить книгу = to buy a book
• открыть книгу = to open the book

Yes, it could be omitted in many contexts.

Russian often drops subject pronouns when the meaning is clear from the verb form:

• (Я) хочу дочитать книгу.

Since хочу already means I want, the pronoun я is not always necessary.

However, keeping я is very normal too. It may be used:

• for clarity
• for emphasis
• because it sounds natural in that particular sentence

So both are possible:

• Перед сном хочу дочитать книгу.
• Перед сном я хочу дочитать книгу.

Russian word order is flexible, and putting перед сном first emphasizes the time frame.

So this sentence highlights when the speaker wants to do it:

• Перед сном я хочу дочитать книгу. = Before bed, I want to finish reading the book.

Other word orders are also possible, for example:

• Я хочу дочитать книгу перед сном.

This means basically the same thing. The difference is mostly in emphasis and style, not core meaning.

By itself, it usually sounds like a current intention or a plan for a specific situation:

• Before bed, I want to finish reading the book.

It does not strongly sound like a repeated habit.

If you wanted a habitual meaning such as Before bed, I like to read, Russian would usually use different wording, for example:

• Перед сном я читаю. = I read before bed.
• Перед сном я люблю читать. = I like to read before bed.

So хочу дочитать suggests a particular desire or intention, not a routine.

Usually finish reading the book.

Because дочитать specifically refers to the action of reading up to the end. It implies that the reading is already in progress or that the book is partly read.

So:

• дочитать книгу = finish reading the book

If you just say read a book, that would normally be прочитать книгу or sometimes читать книгу, depending on the meaning.

A useful contrast:

• читать книгу = read / be reading a book
• прочитать книгу = read a book completely
• дочитать книгу = finish reading the book, especially from where you left off

Because Russian has no articles like a or the.

So книгу can mean:

• a book
• the book

The exact meaning comes from context.

In this sentence, because дочитать means finish reading, English will often naturally use the book, since it sounds like a specific book already being read:

• Before bed, I want to finish reading the book.

But Russian itself does not mark that with an article.

Yes, absolutely.

That version means:

• Before I fall asleep, I want to finish reading the book.

It is a bit more explicit and a little longer than перед сном.

Compare:

• Перед сном я хочу дочитать книгу. = Before bed, I want to finish reading the book.
• Перед тем как уснуть, я хочу дочитать книгу. = Before I fall asleep, I want to finish reading the book.

The original sentence is shorter and very natural in everyday Russian.