Naglo je počela padati kiša, pa smo potrčali kući.

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

In Naglo je počela padati kiša, the first element is Naglo, so the clitic je comes right after it.

So this word order is very natural:

• Naglo je počela padati kiša.

You may also see other orders in Croatian, but learners should get used to this basic rule:

• clitics like je, sam, si, smo, ste, su often appear near the beginning of the clause, in second position.

It contains two parts:

• je počela = the perfect tense of početi (to begin / to start)
• padati = an infinitive (to fall, here used for rain)

So literally, the structure is:

• has started to fall

Croatian often uses:

• početi + infinitive

Examples:

• Počeo je govoriti. = He started speaking.
• Počela je padati kiša. = It started raining.

This is about aspect, which is very important in Croatian.

• padati = imperfective
• pasti = perfective

After početi, Croatian very often uses the imperfective infinitive to describe the beginning of an ongoing action or process.

So:

• počela padati kiša = the rain started falling

That sounds natural because rain is understood as a process.

Using pasti here would sound much less natural in standard usage for this meaning.

Croatian word order is flexible, and the subject does not always have to come before the verb.

Here:

• Naglo je počela padati kiša

the subject kiša comes after the verb phrase. This is very natural in Croatian, especially when introducing an event such as weather beginning suddenly.

You could also say:

• Kiša je naglo počela padati

but that puts more emphasis on kiša as the topic. The original sentence sounds more natural and event-focused.

Because počela agrees with kiša, which is a feminine singular noun.

In the Croatian past tense, the main verb uses the l-participle, which agrees in gender and number with the subject.

Since:

• kiša = feminine singular

you get:

• počela

Compare:

• Vjetar je počeo puhati. = The wind started blowing.
  (vjetar is masculine singular, so počeo)
• Djeca su počela trčati. = The children started running.
  (djeca is plural in agreement, so počela)

Here pa means something like:

• so
• and so
• then

It links the two clauses and shows that the second event happened as a result of the first one:

• it suddenly started raining,
• so we ran home.

It is very common in everyday Croatian.

Because the sentence contains two coordinated clauses:

1. Naglo je počela padati kiša
2. pa smo potrčali kući

In Croatian, a comma is normally used before pa when it connects two full clauses.

So the punctuation here is standard.

Because Croatian usually omits subject pronouns when they are not needed.

The ending and the auxiliary already show the subject:

• smo = we

So:

• smo potrčali already means we ran / we started running

You would add mi only for emphasis or contrast:

• Pa smo potrčali kući. = So we ran home.
• Pa smo mi potrčali kući. = So we ran home. (emphatic)

Because potrčati means to start running / to break into a run, while trčati means to run in a more general or ongoing sense.

So:

• potrčali = we suddenly started running
• trčali = we were running

In this sentence, the idea is that when the rain started, we reacted by breaking into a run, so potrčali is the better choice.

Because it agrees with an implied plural subject: we.

In the past tense, Croatian uses:

• auxiliary + l-participle

Here:

• smo potrčali

The form potrčali is masculine plural or mixed-gender plural, which is the default if the group includes at least one male or if the gender is unspecified.

If the speakers were all female, it would be:

• pa smo potrčale kući

Because kući is a very common directional form meaning home.

Croatian often says:

• ići kući = to go home
• trčati kući = to run home
• vratiti se kući = to return home

So kući works a bit like English home, which also often appears without a preposition:

• go home
• run home

This is different from:

• u kuću = into the house

So:

• kući focuses on home as a destination
• u kuću focuses more literally on entering the building

Historically and grammatically, kući is related to the dative/locative form of kuća, but in modern Croatian learners should mostly remember it as a fixed directional expression used with verbs of motion.

So in practice, just learn:

• kuća = house
• kod kuće = at home
• kući = homeward / home

Examples:

• Idem kući. = I’m going home.
• Došli smo kući. = We came home.

Because Croatian has no articles like English a or the.

So kiša can mean:

• rain
• the rain

The exact meaning depends on context.

In this sentence, English naturally says the rain or simply it started raining, but Croatian does not need an article.

Yes. The whole sentence is in the past.

The past tense is shown by:

• je počela
• smo potrčali

The verb padati stays in the infinitive because it depends on počela:

• started to fall

So only početi is conjugated for tense here, while padati remains the infinitive.

Yes, Croatian word order is flexible, so you could move it, for example:

• Kiša je naglo počela padati.
• Kiša je počela naglo padati.

But the original:

• Naglo je počela padati kiša

sounds very natural and gives a strong sense of suddenness right at the start of the sentence.

Putting naglo first is a common way to emphasize how abrupt the event was.