Čim je počela kiša, potrčali smo kući.

Čim is a conjunction meaning as soon as, the moment, or once.

In this sentence, it introduces a subordinate clause and shows that the second action happened immediately after the first one.

So the structure is:

• Čim + clause = as soon as + clause

Examples:

• Čim sam stigao, nazvao sam te. = As soon as I arrived, I called you.
• Čim je zazvonio telefon, javila se. = As soon as the phone rang, she answered.

Because Čim je počela kiša is a subordinate clause, and potrčali smo kući is the main clause.

Croatian normally separates this kind of clause with a comma, especially when the subordinate clause comes first.

So the sentence is divided like this:

• Čim je počela kiša, = subordinate clause
• potrčali smo kući. = main clause

Because je is a clitic, and Croatian clitics usually go in second position in the clause.

In Čim je počela kiša:

• Čim takes the first position
• je goes right after it

That is why je počela is natural here.

Croatian word order is flexible, but clitic placement is one of the big things that controls it. English speakers often expect a more fixed order, but Croatian does not work that way.

Because kiša is a feminine singular noun, and the past participle must agree with the subject in gender and number.

So:

• kiša = feminine singular
• therefore počela = feminine singular

Compare:

• Počeo je film. = The film started.
  
  • film is masculine singular
• Počela je kiša. = The rain started.
  
  • kiša is feminine singular
• Počelo je nevrijeme. = The storm/bad weather started.
  
  • nevrijeme is neuter singular

Yes. Kiša is the subject.

Croatian word order is much freer than English word order, so the subject does not always have to come before the verb.

In English, the rain started is the normal order. In Croatian, both grammar and meaning can still be clear even when the subject comes later:

• Počela je kiša.

You can recognize kiša as the subject because:

• it is in the nominative form
• the participle počela agrees with it in feminine singular

It is the Croatian perfect tense.

The perfect is formed with:

• the present tense of biti = je
• the l-participle = počela

So:

• je počela = has started / started

In everyday Croatian, the perfect is the normal way to talk about past events.

Because potrčati and trčati are not the same in aspect or meaning.

• trčati = to run, to be running
  
  • imperfective
• potrčati = to start running, to run off suddenly
  
  • perfective

In this sentence, potrčali smo suggests a quick reaction: the rain started, and we immediately broke into a run toward home.

If you said trčali smo kući, it would sound more like we were running home or we ran home with less emphasis on the sudden start.

So potrčali smo fits very well with čim.

Again, this is because smo is a clitic.

Clitics usually go in second position in the clause. In the clause potrčali smo kući:

• potrčali occupies the first position
• smo goes right after it

So potrčali smo is the normal order here.

If you added the pronoun mi, then you could get:

• Mi smo potrčali kući.

There, mi takes first position, and smo goes after it.

Because Croatian usually does not need a subject pronoun when the verb already shows who is doing the action.

In potrčali smo:

• smo marks we
• potrčali is plural masculine/mixed form, matching that subject

So mi is unnecessary unless you want emphasis or contrast.

Compare:

• Potrčali smo kući. = We ran home.
• Mi smo potrčali kući, a oni su ostali vani. = We ran home, but they stayed outside.

In the second sentence, mi is added for contrast.

Kući means home in the sense of toward home.

It is a special, very common form used with verbs of motion:

• ići kući = to go home
• doći kući = to come home
• trčati kući = to run home

Although it comes from kuća, learners should mostly treat kući as a set expression meaning homeward / to home.

Compare:

• Idem kući. = I’m going home.
• Ušao sam u kuću. = I entered the house.

So:

• kući = home, as a destination
• u kuću = into the house, stressing entry into the building

Because the sentence describes two completed, point-like events in sequence:

1. the rain started
2. we ran off / started running home

Perfective verbs are very natural here because the sentence is about the beginning of one event triggering the immediate beginning of another.

This gives the sentence a sharp, sudden feeling:

• rain starts
• instant reaction

That matches čim very well.

Yes, Croatian allows some variation, but the original sentence is very natural.

Possible variations include:

• Čim je počela kiša, kući smo potrčali.
• Potrčali smo kući čim je počela kiša.

These are grammatically possible, but they may sound more marked or place emphasis differently.

The original:

• Čim je počela kiša, potrčali smo kući.

is the most neutral and natural version for everyday use.

Yes, very much so.

Croatian often uses:

• Počela je kiša. = It started raining / The rain started.

It is a standard, idiomatic way to talk about the weather.

You may also hear:

• Počela je padati kiša. = It started to rain

Both are natural. The version in your sentence is shorter and very common.