Onda sam bila bolesna.

It breaks down like this:

• Onda = then, at that time
• sam = I am in its clitic form, but here it helps make the past tense
• bila = been / was feminine past participle of biti (to be)
• bolesna = sick / ill feminine adjective

So the structure is roughly:

• Onda
  • sam bila
    • bolesna
• Then
  • I was
    • sick

In natural English, this is simply Then I was sick or At that time I was sick.

Because Serbian commonly forms the past tense with:

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

So sam bila means I was.

For the verb biti, the past tense works like this:

• ja sam bio / bila = I was
• ti si bio / bila = you were
• on je bio = he was
• ona je bila = she was
• mi smo bili / bile = we were

So even though sam by itself is a present form, in combination with bila it creates the past tense.

Because bila is the feminine singular form.

In Serbian, the past participle agrees with the gender of the speaker or subject:

• bio = masculine singular
• bila = feminine singular
• bilo = neuter singular
• bili = masculine plural or mixed group
• bile = feminine plural

So:

• a woman says sam bila
• a man says sam bio

This sentence tells you that the speaker is female.

Because adjectives in Serbian must agree with the noun or subject in:

• gender
• number
• case

Here the implied subject is I, and the speaker is female, so the adjective is feminine singular:

• bolestan = masculine
• bolesna = feminine
• bolesno = neuter

So a male speaker would say:

• Onda sam bio bolestan.

A female speaker says:

• Onda sam bila bolesna.

In Serbian, subject pronouns are often omitted because the verb form already tells you the person.

Here, sam clearly shows I. So ja is not necessary.

The normal sentence is:

• Onda sam bila bolesna.

You can add ja for emphasis or contrast, but then word order has to be handled carefully:

• Onda sam ja bila bolesna. = Then I was the one who was sick.
• Ja sam onda bila bolesna. = I was sick then.

But Onda ja sam bila bolesna sounds unnatural, because sam is a clitic and usually wants to be in second position.

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

So in:

• Onda sam bila bolesna

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

This is a very important Serbian pattern.

Compare:

• Juče sam bila kod kuće. = Yesterday I was at home.
• Tada sam bila umorna. = Then / at that time I was tired.
• Kasnije sam došla. = Later I came.

The clitic usually cannot just go anywhere.

No. Serbian word order is flexible, but clitic placement still matters.

Possible versions include:

• Onda sam bila bolesna.
• Ja sam onda bila bolesna.
• Bila sam bolesna onda. — possible, but less neutral and more marked
• Onda sam ja bila bolesna. — emphasizes I

The most neutral version is the original:

• Onda sam bila bolesna.

So onda does not have to be first, but when it is first, sam naturally follows it.

Onda can mean several related things depending on context:

• then in a sequence: First this happened, then that happened
• at that time: referring to a period in the past
• sometimes in that case in other contexts

In Onda sam bila bolesna, it most often means:

• then
• or at that time

The exact nuance depends on the wider context.

For example:

• Prvo sam radila, onda sam bila bolesna. = First I was working, then I was sick.
• Nisam mogla da dođem, jer sam onda bila bolesna. = I couldn’t come, because I was sick at that time.

Yes, though they often overlap.

• onda = then, at that time; very common in both speech and writing
• tada = then, at that time; often a bit more formal or explicit
• tad = shortened form of tada, very common in speech

In many contexts they can replace one another:

• Onda sam bila bolesna.
• Tada sam bila bolesna.
• Tad sam bila bolesna.

These can all mean roughly the same thing.
The differences are mostly about style, rhythm, and context.

This is the perfect tense in Serbian, which is the main everyday past tense.

It is formed with:

• present tense of biti as an auxiliary
• plus the active past participle

Here:

• sam = auxiliary
• bila = participle

So:

• sam bila = I was

Serbian uses this tense very broadly for past actions and states, much more like English simple past in many cases.

A man would say:

• Onda sam bio bolestan.

Changes:

• bila → bio
• bolesna → bolestan

Both the participle and the adjective change to masculine.

No. If you are male, both parts should match masculine gender:

• sam bio bolestan

Using sam bila bolesna identifies the speaker as female.

In Serbian, this kind of gender agreement is not optional in standard language.

Usually bolestan / bolesna means ill or sick, and in sentences like this it normally refers to a state at that time.

So:

• Onda sam bila bolesna = At that time I was sick / ill

It does not necessarily say how serious the illness was or how long it lasted. Context tells you that.

Yes, depending on meaning:

• bolesna = sick, ill
• slaba = weak
• prehlađena = having a cold
• umorna = tired

But if the intended meaning is specifically sick/ill, then bolesna is the normal choice.

They often stay together, but words can sometimes come between them, depending on emphasis and sentence structure.

For example:

• Onda sam bila bolesna. — neutral
• Onda sam ja bila bolesna. — emphasis on I
• Ja sam tada bila bolesna. — also normal

So sam is tightly tied to second position, but bila does not always have to come immediately after it. Still, in a simple sentence, keeping sam bila together is the easiest and most natural pattern for learners.

Yes.

• Bila sam bolesna. = I was sick.
• Onda sam bila bolesna. = Then / at that time I was sick.

Adding onda gives a time reference. Without it, the sentence is just a plain statement about the past.

Both are correct.