Ja sam već potpisala obrazac, a sada čekam potvrdu.

Because Serbian past tense agrees with the gender of the speaker.

In Ja sam već potpisala obrazac, potpisala shows that the speaker is female.

• potpisala = a woman says I signed / I have signed
• potpisao = a man says I signed / I have signed

So:

• Ja sam već potpisala obrazac = said by a woman
• Ja sam već potpisao obrazac = said by a man

This is very common in Serbian past tense.

Serbian past tense is often made with:

• the present tense of biti (to be)
• plus a past participle

So sam potpisala literally consists of:

• sam = I am
• potpisala = signed

Together, they mean I signed or I have signed, depending on context.

This is the normal way to form the past tense in Serbian:

• ja sam došla = I came / I have come
• ja sam videla = I saw / I have seen
• ja sam potpisala = I signed / I have signed

Yes, ja can be omitted.

Serbian usually does not need subject pronouns, because the verb form already shows the person:

• sam potpisala already means I signed / I have signed

So a very natural version is:

• Već sam potpisala obrazac, a sada čekam potvrdu.

You include ja when you want:

• extra emphasis
• contrast
• clarity

For example, Ja sam već potpisala... can sound like I have already signed it.

Because sam is a clitic, and Serbian clitics usually go in the second position of the sentence or clause.

So in:

• Ja sam već potpisala obrazac

the first element is Ja, and sam comes right after it.

If you remove ja, a natural order is:

• Već sam potpisala obrazac.

Noticing clitic position is very important in Serbian. Similar clitics include:

• sam, si, je, smo, ste, su
• se
• short object pronouns like ga, je, mi, ti

Već means already.

So:

• već potpisala = already signed

It shows that the action happened before now, often with the sense that it is completed sooner than expected or before some next step.

Examples:

• Već sam jela. = I have already eaten.
• Već znam. = I already know.

It actually is in the accusative, but for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

Here:

• obrazac = form
• after potpisati (to sign), it is the direct object
• direct objects usually go in the accusative

So grammatically this is accusative, even though the form stays:

• nominative: obrazac
• accusative: obrazac

This happens because obrazac is masculine and inanimate.

Because potvrdu is the accusative singular of potvrda.

• nominative: potvrda = confirmation
• accusative: potvrdu

After čekam (I am waiting for), the thing being waited for is the direct object, so Serbian uses the accusative:

• čekam potvrdu = I am waiting for the confirmation

This is a very common pattern with feminine nouns ending in -a:

• knjiga → knjigu
• poruka → poruku
• potvrda → potvrdu

Because Serbian present tense often covers both:

• I wait
• I am waiting

So:

• čekam can mean I wait or I am waiting
• with sada (now), it clearly means I am waiting now

That is why:

• a sada čekam potvrdu = and now I am waiting for the confirmation

Serbian does not need a separate progressive form like English am waiting.

In this sentence, a means something like:

• and
• while
• and now / whereas

It often connects two clauses with a slight contrast or shift of focus.

So:

• Ja sam već potpisala obrazac, a sada čekam potvrdu.

means something like:

• I have already signed the form, and now I’m waiting for the confirmation.

Here a works well because the speaker moves from one stage to the next:

• first: signed the form
• now: waiting for confirmation

If you used i, it would sound more like simple addition. A is more natural here because it marks a transition.

Yes, mostly.

• sada = now
• sad = now

Sad is just a shorter, very common form. In everyday speech, sad is extremely frequent.

So both are natural:

• a sada čekam potvrdu
• a sad čekam potvrdu

Sada can sound slightly more neutral or a bit more careful, while sad is often more conversational.

Yes. The verb here is from potpisati, which is a perfective verb, so it presents the signing as a completed action.

That fits the meaning very well:

• already signed the form
• the action is done
• now the speaker is at the next step

By contrast, čekam comes from čekati, an imperfective verb, because waiting is viewed as an ongoing process.

So the sentence combines:

• completed action: sam potpisala
• ongoing action/state: čekam

This is a very typical Serbian aspect contrast.

Yes, Serbian word order is fairly flexible, but some versions are more natural than others.

Very natural versions include:

• Već sam potpisala obrazac, a sada čekam potvrdu.
• Ja sam već potpisala obrazac, a sada čekam potvrdu.

The first one is often more natural in everyday Serbian because the pronoun ja is not necessary.

You can change word order for emphasis, but you must still respect normal clitic placement, especially with sam. For example:

• Već sam potpisala obrazac is natural
• Sam već potpisala obrazac is not natural as a standalone sentence

So yes, word order can move, but not completely freely.