Naručila sam knjigu.

Because the speaker is female.

In Serbian past tense, the main past form agrees with the subject’s gender and number:

• naručila sam = I ordered (said by a woman)
• naručio sam = I ordered (said by a man)

So Naručila sam knjigu. is specifically something a woman would say.

sam is the auxiliary verb to be, used to form the past tense.

Serbian past tense is usually built like this:

• past participle + auxiliary

So here:

• naručila = past participle
• sam = am, the 1st person singular form of biti (to be)

Together, naručila sam means I ordered.

Because sam is a clitic in Serbian. Clitics are short unstressed words that usually cannot stand at the beginning of a sentence and normally go in the second position.

So this is natural:

• Naručila sam knjigu.

But this is not standard:

• Sam naručila knjigu.

You can also say:

• Ja sam naručila knjigu.

Here ja is the first element, and sam still comes in second position.

Because knjigu is in the accusative singular, which is used for the direct object.

The basic form is:

• knjiga = book

But after a verb like order, the noun is the thing being ordered, so it becomes the direct object:

• knjigu = book in the accusative

For many feminine nouns ending in -a, the accusative singular ends in -u:

• knjiga → knjigu
• kafa → kafu
• ulaznica → ulaznicu

The dictionary form is naručiti.

That is the infinitive, meaning to order.

So the forms are:

• naručiti = to order
• naručila sam = I ordered (female speaker)
• naručio sam = I ordered (male speaker)

It is perfective.

That means it presents the action as completed: the order was placed as a whole, finished action.

So Naručila sam knjigu. suggests:

• the action happened
• it was completed

The imperfective partner is usually naručivati, which is used for repeated, ongoing, or habitual ordering.

Compare:

• Naručila sam knjigu. = I ordered a book. / I have ordered a book.
• Naručivala sam knjige. = I used to order books. / I was ordering books.

Because Serbian often drops subject pronouns when they are not needed.

English usually requires I, but Serbian does not. The verb phrase already tells you the person, and in the past tense it can also show gender:

• naručila sam already clearly means I ordered and shows the speaker is female

So ja is often omitted unless you want emphasis or contrast:

• Naručila sam knjigu. = neutral
• Ja sam naručila knjigu. = I ordered the book, not someone else

Yes. Serbian word order is fairly flexible, though some versions sound more neutral than others.

Neutral:

• Naručila sam knjigu.

Also possible:

• Ja sam naručila knjigu.
• Knjigu sam naručila.

The meaning stays broadly the same, but the emphasis changes:

• Ja sam naručila knjigu. emphasizes I
• Knjigu sam naručila. emphasizes the book

So word order in Serbian often helps show focus rather than changing the basic meaning.

By itself, knjigu does not automatically distinguish between a book and the book the way English does.

Serbian has no articles like a and the.

So Naručila sam knjigu. could mean:

• I ordered a book
• I ordered the book

The exact meaning depends on context.

If the book is already known from the conversation, English would often use the book.
If it is new information, English might use a book.

A rough pronunciation guide is:

• Naručila ≈ nah-ROO-chee-lah
• sam ≈ sahm
• knjigu ≈ KNYEE-goo

A few helpful notes:

• č sounds like ch in church
• nj sounds like ny in canyon
• g is always a hard g, like in go

So the whole sentence is roughly:

• nah-ROO-chee-lah sahm KNYEE-goo

Sometimes yes, but it depends on context and regional usage.

• naručiti often means to order in the sense of placing an order
• poručiti can also mean to order, but it can additionally mean to send a message / tell someone

In many situations, naručila sam knjigu is the clearest choice for I ordered a book.

So for a learner, naručiti is a very safe and common verb to use for ordering something.