За неделю я прочитал новую книгу.

Russian prepositions often have several meanings depending on context and case.

In За неделю я прочитал новую книгу, за + accusative can express the time needed to complete something:

• за неделю = in a week / within a week
• It suggests the action was finished by the end of that time period.

So here it does not mean physical behind. It means something like:

• I finished reading the book in a week
• It took me a week to read the book

This is a very common pattern:

• за час = in an hour
• за два дня = in two days
• за месяц = in a month

Неделю is accusative singular of неделя.

That happens because the preposition за in this time-expression pattern takes the accusative:

• nominative: неделя
• accusative: неделю

So:

• за неделю
• за месяц
• за год
• за два дня

This use of the accusative is tied to the idea of completing something within a span of time.

This is about aspect, one of the most important parts of Russian verbs.

• читать = imperfective
• прочитать = perfective

In this sentence, прочитал is used because the action is presented as completed. The whole book was read from beginning to end.

So:

• Я читал новую книгу. = I was reading a new book / I read a new book (without focusing on completion)
• Я прочитал новую книгу. = I read/finished a new book

Because the sentence includes за неделю and describes finishing something within a certain time, the perfective verb is the natural choice.

It adds the idea of a result or completed whole action.

With прочитал, the sentence implies:

• the reading is finished
• the book was completed
• the time span (за неделю) led to a finished result

That is why за неделю works especially well with perfective verbs. It often answers the question:

• How long did it take to complete this?

So the sentence is not just about the process of reading, but about the completed achievement.

Russian past tense does not use an extra helper verb like English often does.

Instead, Russian forms the past tense with a past-tense form of the verb itself:

• прочитал = read / finished reading

There is no separate word for did or have here.

Also, in the past tense, Russian verbs agree with gender and number, not person:

• я прочитал = I read (male speaker)
• я прочитала = I read (female speaker)
• мы прочитали = we read

So this sentence sounds as if the speaker is male. If the speaker is female, it would be:

• За неделю я прочитала новую книгу.

Because книга is the direct object of the verb, so it goes into the accusative case.

Base form:

• новая книга = a new book

Accusative singular:

• новую книгу

Both the adjective and noun change to match:

• новая → новую
• книга → книгу

So:

• nominative: новая книга
• accusative: новую книгу

This is normal for feminine nouns ending in -а and their adjectives.

Russian has no articles, so there is no direct equivalent of a or the.

That means новую книгу could mean:

• a new book
• the new book

The exact interpretation depends on context.

Without extra context, English often translates it as a new book. But if both speaker and listener already know which book is meant, the new book is also possible.

So the Russian sentence itself does not mark that difference.

Yes. Russian word order is more flexible than English word order because case endings show grammatical roles.

The sentence:

• За неделю я прочитал новую книгу.

is perfectly natural. It puts за неделю first, which gives that time expression some emphasis.

Other possible orders:

• Я прочитал новую книгу за неделю.
• Я за неделю прочитал новую книгу.
• Новую книгу я прочитал за неделю.

These all express basically the same core idea, but the focus shifts slightly:

• sentence-initial За неделю highlights the time span
• sentence-initial Новую книгу highlights the object
• more neutral order is often Я прочитал новую книгу за неделю

Here it means in a week / within a week, not simply at some time during the week.

That is an important distinction.

• за неделю = in a week, over the course of one week, by the end of that week the action was completed
• на неделе = during the week / sometime this week
• в течение недели = during the course of the week / throughout the week

So in this sentence, за неделю tells you how long it took to finish reading the book.

You could say Я читал новую книгу неделю, but it would mean something different.

Compare:

• За неделю я прочитал новую книгу.
  = I finished reading the new book in a week.

• Я неделю читал новую книгу.
  = I was reading the new book for a week.

The second sentence focuses on the duration of the activity, not necessarily the completed result.

So читал is not exactly wrong, but it does not express the same idea as прочитал.