Juče sam zaboravila papir u kancelariji.

Because sam is a clitic in Serbian. Clitics are short unstressed words that usually go in second position in the clause.

So in:

Juče sam zaboravila papir u kancelariji.

the first element is Juče and the clitic sam comes right after it.

This often feels unusual to English speakers, because English keeps auxiliaries more tightly attached to the main verb. In Serbian, though, second-position clitic placement is very normal.

Compare:

• Juče sam zaboravila papir.
• U kancelariji sam zaboravila papir.
• Papir sam zaboravila juče.

In all of these, sam tends to appear after the first stressed element or phrase.

Because zaboravila is the form used in the past tense, while zaboraviti is the infinitive.

Serbian past tense is usually formed with:

• a present-tense form of biti (sam, si, je, smo, ste, su)
• plus the l-participle

So here:

• sam = I have / am as the auxiliary
• zaboravila = past participle of zaboraviti

Together, sam zaboravila means I forgot.

That ending shows the gender of the speaker in the past tense.

In Serbian, past-tense participles agree with the subject in gender and number.

For I forgot, you get:

• zaboravio sam = if the speaker is male
• zaboravila sam = if the speaker is female

So this sentence tells you that the speaker is female.

Serbian often drops subject pronouns when they are not needed, because the verb form already shows the person.

In sam zaboravila, it is already clear that the subject is I.

So:

• Sam zaboravila papir. is not possible by itself because sam needs proper placement,
• but Juče sam zaboravila papir... is perfectly normal without ja.

You can add ja for emphasis:

• Ja sam zaboravila papir u kancelariji.

That would sound more like I’m the one who forgot the paper in the office.

Papir is the direct object, so it is in the accusative case. But for many inanimate masculine singular nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: papir
• accusative: papir

That is normal.

Compare with an animate masculine noun, where accusative changes:

• nominative: student
• accusative: studenta

So papir is in the accusative here, even though the form does not change.

Here u kancelariji is in the locative case, because it means location: where something happened or where something was.

• kancelarija = base form
• u kancelariji = in the office

With u, Serbian uses:

• locative for location: u kancelariji = in the office
• accusative for motion toward/into something: u kancelariju = into the office

So:

• Zaboravila sam papir u kancelariji. = I forgot the paper in the office
• Unela sam papir u kancelariju. = I brought the paper into the office

Because u can express either:

1. location → use locative
2. direction/movement into → use accusative

In this sentence, the meaning is static location, so Serbian uses locative:

• u kancelariji

This is a very important pattern in Serbian and other Slavic languages.

Examples:

• Sedim u sobi. = I am sitting in the room. → location, so locative
• Ulazim u sobu. = I am entering the room. → movement into, so accusative

Yes. Serbian word order is fairly flexible, though not completely free. Different orders can change focus or emphasis.

Your sentence:

• Juče sam zaboravila papir u kancelariji.

Other possible versions include:

• Zaboravila sam papir u kancelariji juče.
• Papir sam juče zaboravila u kancelariji.
• U kancelariji sam juče zaboravila papir.

These all keep roughly the same basic meaning, but they highlight different parts of the sentence.

Still, Juče sam zaboravila papir u kancelariji is a very natural neutral sentence.

Because of the usual placement of clitics like sam.

In a normal declarative clause, sam wants to be in second position, not later after the main verb if something else already comes first.

So:

• Juče sam zaboravila papir... ✅
• Juče zaboravila sam papir... ❌ in standard neutral Serbian

But if the participle comes first, then sam can follow it:

• Zaboravila sam papir u kancelariji. ✅

That works because zaboravila is now the first element, and sam is still in second position.

Zaboraviti is perfective. It describes a completed event: the moment or fact of forgetting.

That fits this sentence well, because the speaker is referring to a completed past action:

• Juče sam zaboravila papir... = Yesterday I forgot the paper...

The imperfective partner is usually zaboravljati. That would suggest repeated, habitual, or ongoing forgetting in the right context.

For example:

• Često zaboravljam papire u kancelariji. = I often forget papers in the office.

So in your sentence, the perfective verb is the natural choice.

Serbian does not have articles like English a/an and the.

So papir by itself can mean:

• a paper
• the paper

Which one is intended depends on context.

That is very normal in Serbian. Speakers usually understand from the situation whether the noun is specific or not.

If needed, Serbian can add other words for clarity, such as:

• taj papir = that paper
• jedan papir = one / a certain paper

But in many sentences, plain papir is enough.

It does not have to be first. Serbian often puts time expressions early in the sentence, but that is a stylistic and informational choice, not a strict rule.

You could also say:

• Papir sam zaboravila juče u kancelariji.
• Zaboravila sam juče papir u kancelariji.

Putting Juče first is very common when the speaker wants to establish the time immediately.

The dictionary form is kancelarija.

Here is the relevant pattern:

• nominative singular: kancelarija
• locative singular: kancelariji

So after u meaning location, you get:

• u kancelariji

This is a common pattern for many feminine nouns ending in -a.

A man would need to change the participle:

• female speaker: Juče sam zaboravila papir u kancelariji.
• male speaker: Juče sam zaboravio papir u kancelariji.

Everything else stays the same.

So yes, this exact sentence is specifically from the point of view of a female speaker.

A useful breakdown is:

• Juče = yesterday
• sam zaboravila = I forgot
• papir = paper / the paper
• u kancelariji = in the office

That shows the structure clearly:

Time + auxiliary + past participle + object + place

Even though Serbian word order is flexible, this is a very natural and learner-friendly way to read the sentence.