Questions & Answers about 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: