Learn Hardcore Croatian: Da sam ranije čula za tu peticiju, potpisala bih je već jučer. - If I had heard about that petition earlier, I would have signed it already yesterday.

As a complementizer meaning that (e.g. Rekla je da je umorna. – She said that she is tired.)
As a conditional subordinator meaning if, especially in unreal or hypothetical conditions referring to the past.

In your sentence:

Da sam ranije čula za tu peticiju, ...
→ da = if (unreal condition in the past: “If I had heard earlier…”)

So here da introduces an unreal/contrary-to-fact condition. You could also say:

Kad bih ranije čula za tu peticiju, potpisala bih je odmah. (more like a general hypothetical)
Ako sam ranije čula … is not used for this type of unreal past condition; ako is usually for real or possible conditions, mostly present/future-oriented.

For “If I had heard earlier…”, Croatian strongly prefers da + past perfect (da sam čula), not ako.

What tense is sam ranije čula? Is it like English past perfect had heard?

Sam ranije čula is the perfect tense (past) formed with the auxiliary biti (sam) + the past participle (čula).

čuti – to hear
(ja) sam čula – I heard / I have heard

In this construction:

sam = 1st person singular of biti (to be), used as a past auxiliary
čula = past participle, feminine, agreeing with the (implied) feminine subject ja

In this specific sentence, because it’s under da and part of a conditional structure, da sam čula corresponds very closely to English if I had heard (past unreal condition), even though formally it’s just the normal perfect. The whole pattern:

Da sam ranije čula… ≈ If I had heard earlier…

So functionally it behaves like an English past perfect in conditional sentences, even if the formal label in Croatian is just “perfect (perfekt)”.

Why is it čula and potpisala and not čuo / potpisao?

The past participle in Croatian always agrees with the gender and number of the subject:

Masculine singular: čuo, potpisao
Feminine singular: čula, potpisala
Neuter singular: čulo
Plural (mixed or masculine group): čuli, etc.

Your sentence assumes the speaker is female:

(ja) sam čula – I (female) heard
(ja) bih potpisala – I (female) would have signed

If the speaker were male, it would be:

Da sam ranije čuo za tu peticiju, potpisao bih je već jučer.

So the change in the ending (-a vs -o) signals the speaker’s gender.

What exactly does bih do in potpisala bih je? Is this a future tense?

Bih is the conditional auxiliary (1st person singular of the conditional mood of biti: I would).

The pattern is:

bih – I would
bi – you (sg.) / he / she / it would
bismo – we would
biste – you (pl.) would
bi – they would

Conditional is formed as:

bih
- past participle (L-participle): bih potpisala
  → I would sign / I would have signed

In this sentence:

potpisala bih je = I would have signed it (because the whole structure refers to the past).

So:

sam čula – I heard
bih potpisala – I would have signed / I would sign (context tells it’s past unreal here)

This is not a future tense; Croatian future uses ću + infinitive (potpisat ću = I will sign).

Why is it potpisala bih je and not bih potpisala je? Where do bih and je go in the sentence?

Croatian has clitics (short unstressed words) like sam, bih, je, ga, mi, ti, se that tend to cluster in the second position in their clause.

In your clause potpisala bih je već jučer, the natural order is:

The first stressed word: potpisala
Then the cluster of clitics: bih je
Then the rest: već jučer

So we get:

Potpisala bih je već jučer.

You will not normally say:

✗ Bih potpisala je već jučer. (ungrammatical)

Other possible word orders are more limited and still keep clitics early, for example:

Već bih je jučer potpisala.
(Here the first stressed word is Već, then comes the clitic cluster bih je.)

The biggest takeaway: bih and je are clitics and must stay very early in the clause, right after the first stressed element.

What does je refer to in potpisala bih je? Why is it je and not something else?

Je here is a direct object pronoun in the accusative, feminine singular:

je / ju = her / it (for feminine singular nouns)
It refers back to tu peticiju (that petition), which is feminine singular.

So:

(tu peticiju) → feminine singular → pronoun je (or ju) in accusative
Potpisala bih je = I would have signed it (the petition).

Other relevant forms for comparison:

Masculine: ga (him / it – masculine)
Neuter: usually ga as well in spoken language
Plural (any gender): ih (them).

Can I use ju instead of je (e.g. potpisala bih ju)?

Yes, in many dialects and in informal speech you will hear:

Potpisala bih ju.

Je and ju are both feminine singular accusative pronouns for her / it. Usage depends on region and register:

je – more standard, very common everywhere
ju – also correct, but more regional/colloquial in many places

In writing and in standard language teaching, je is usually given as the main option, which is why your sentence uses je.

Why is it čuti za tu peticiju and not čuti tu peticiju or čuti o toj peticiji?

The verb čuti (to hear) often combines with za + accusative to mean hear about (the existence of) something:

čuti za nešto – hear about something, become aware it exists
- Čula sam za tu peticiju. – I’ve heard about that petition.

Nuances:

čuti nešto – hear something as a sound or piece of information:
- Čula sam glasnu muziku. – I heard loud music.
- Čuo sam vijest. – I heard the news (as a specific piece of info).
čuti o nečemu – hear (some information) about a topic; more like “hear details about”:
- Čula sam o toj peticiji. – I heard about that petition (some news/info about it).

In practice, for “I had heard of that petition / heard about that petition (that it exists)”, čuti za is the most idiomatic: čuti za tu peticiju.

What exactly does tu in tu peticiju mean? How is it different from ovu or onu?

Croatian has three basic demonstratives, often described as:

ova (this – near the speaker)
ta / tu (that – near the listener or relatively close)
ona (that – farther away, or more distant in context)

Forms in accusative feminine singular (as in your sentence):

ovu peticiju – this petition (here, near me / very psychologically close)
tu peticiju – that petition (near you / just mentioned / relatively close)
onu peticiju – that petition over there / that one we talked about a while ago / more distant

In your sentence, tu peticiju suggests a petition already known in the conversation, but not strongly emphasized as “this right here”. English usually just says that petition (or that petition you mentioned).

Why is there a comma after peticiju? Is it mandatory?

Yes, the comma is mandatory here. Croatian separates clauses of a conditional sentence with a comma, especially when the if-clause comes first:

Da sam ranije čula za tu peticiju, potpisala bih je već jučer.
Ako dođeš, javim ti se.
Kad bih imao više vremena, učio bih hrvatski svaki dan.

If you reverse the order, you still usually keep the comma:

Potpisala bih je već jučer, da sam ranije čula za tu peticiju.

So, in this kind of sentence da …, (main clause), the comma is standard and expected.

What does već jučer add? Why not just jučer?

jučer on its own = yesterday
već jučer = already yesterday / as early as yesterday

Već adds a nuance of earliness or sooner than you might expect. So:

Potpisala bih je jučer. – I would have signed it yesterday.
Potpisala bih je već jučer. – I would have signed it already yesterday / I’d have signed it as early as yesterday (not later).

It emphasizes that the signing would have happened immediately once I knew, not at some vague later point.

Could I change the word order, like Da sam ranije za tu peticiju čula or Već bih je jučer potpisala?

Yes, Croatian allows quite a lot of word order variation, as long as you respect clitic placement and the meaning stays clear. Some possible variants:

Conditional clause:
- Da sam ranije čula za tu peticiju, … (neutral, standard)
- Da sam ranije za tu peticiju čula, … (slightly emphasizing za tu peticiju)
- Da sam za tu peticiju ranije čula, … (emphasis on for that petition and earlier)
Main clause:
- Potpisala bih je već jučer. (neutral, standard)
- Već bih je jučer potpisala. (emphasizes već = already)
- Jučer bih je već potpisala. (emphasis on jučer)

In all of them:

Clitics (bih, je, sam) must remain very early in the clause, after the first stressed element.
No matter the word order, the basic meaning remains the same; changes are mostly about focus and emphasis.

Is there a more “literal” way to show the structure in English so I can remember the pattern?

You can think of the Croatian structure as:

Da
- [I am having-heard earlier] about that petition, [I would-have signed it already yesterday].

More structurally:

Da
- sam (have/was) + čula (heard) → “If I had heard”
potpisala bih je = “I would have signed it”

So the mental template:

Da sam [verb-participle], [verb-participle] bih [object] [time].
- Da sam ranije čula za to, potpisala bih je odmah.
- Da sam znao, rekao bih ti. – If I had known, I would have told you.

Remember: da + perfect (sam čula) in the first clause, and conditional (bih + participle) in the second clause is the standard way to say “If I had done X, I would have done Y” in Croatian.