Fioka koju smo poravnali sada je stabilna.

Because koju is a relative pronoun, and its case depends on its role inside the relative clause.

• fioka is feminine singular
• inside koju smo poravnali, that pronoun is the direct object of poravnali
• so it must be feminine singular accusative: koju

If it were the subject of the relative clause, you would use koja instead.

It is a relative clause: it describes fioka.

The sentence can be broken down like this:

• Fioka ... je stabilna. = The drawer ... is stable.
• koju smo poravnali = that/which we aligned

So the full structure is:

• Fioka = the noun
• koju smo poravnali = extra information about that noun
• sada je stabilna = the main statement about it now

It is the perfect tense in Serbian:

• smo = auxiliary verb to be for we
• poravnali = past participle

So literally it is something like we have aligned, but in normal English it is often best translated as simple past: we aligned.

Serbian uses this tense very often where English would simply use the past tense.

Because the participle agrees with the implied subject mi = we.

• poravnali = masculine plural, or the default plural form for a mixed group / unspecified gender
• poravnale = all-female we

So this sentence either assumes a male or mixed group, or just uses the default plural form.

Because adjectives in Serbian agree with the noun they describe.

Since fioka is:

• feminine
• singular
• nominative

the adjective also has to be feminine singular nominative:

• stabilna

Compare:

• stabilan = masculine singular
• stabilna = feminine singular
• stabilno = neuter singular

They are linked to the same thing, but they have different jobs in different clauses.

• fioka is nominative singular because it is the subject of the main clause
• koju is accusative singular because in the relative clause it functions as the direct object of poravnali

This is a very common pattern in Serbian relative clauses.

Because Serbian has no articles like English the or a/an.

So fioka can mean:

• a drawer
• the drawer

The context tells you which one is meant. In this sentence, it will usually be understood as the drawer.

Because Serbian word order is flexible, and short forms like je often move around depending on rhythm and emphasis.

In this sentence:

• sada je stabilna sounds very natural

You could also say:

• Fioka koju smo poravnali je sada stabilna.

That version is also correct. The difference is mostly in flow and emphasis, not basic meaning.

Yes, to some extent.

For example, these are both natural:

• Fioka koju smo poravnali sada je stabilna.
• Fioka koju smo poravnali je sada stabilna.

Serbian allows more flexibility than English, but the relative clause koju smo poravnali normally stays close to fioka, because it modifies that noun.

Yes, essentially.

• sada = full form
• sad = shorter everyday form

Both mean now.
Sada can sound a bit more careful or neutral in writing, while sad is extremely common in speech.

A rough pronunciation guide would be:

• Fioka = fee-OH-ka
• koju = KO-yu
• smo = smo
• poravnali = po-RAV-na-li
• sada = SA-da
• je = ye
• stabilna = sta-BEEL-na

A very rough full reading:

fee-OH-ka KO-yu smo po-RAV-na-li SA-da ye sta-BEEL-na

This is only an approximation, but it is a useful starting point for an English speaker.