Samo što smo stigli kući, počela je kiša.

In this sentence, samo što is a fixed expression meaning just after, as soon as, or no sooner had... than.

So:

• Samo što smo stigli kući, počela je kiša.
• = We had just arrived home when it started raining.

This is not the literal only that meaning. Croatian samo što can mean different things depending on context, and here it introduces an event that happened almost immediately after another one.

A useful way to remember it here is:

• samo što + past action = just after / hardly had ... when

Both are in the Croatian perfect tense.

The perfect is formed with:

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

So here:

• smo stigli = we arrived / we have arrived
• je počela = started / has started

In Croatian, this tense is the normal way to talk about completed past events in everyday language.

Breakdown:

• smo = we are auxiliary, used to build the perfect
• stigli = past participle of stići (to arrive), masculine plural because we includes at least one male or is mixed/unspecified
• je = is auxiliary, used to build the perfect
• počela = past participle of početi (to begin/start), feminine singular because it agrees with kiša

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

In the clause:

• Samo što smo stigli kući

the whole expression Samo što counts as the first unit, so the clitic smo comes right after it.

That is why you get:

• Samo što smo stigli kući...

not normally:

• Samo što stigli smo kući...

The same thing happens in the second clause:

• počela je kiša

Here je is also a clitic, so it goes in second position in its clause.

This clitic placement is one of the most important word-order rules in Croatian.

Both are possible, but počela je kiša is a very natural Croatian word order.

Croatian word order is more flexible than English, and speakers often move words around for rhythm, focus, or style. The clitic je still stays in second position, so:

• Počela je kiša.
• Kiša je počela.

Both mean The rain started or It started raining.

In this sentence, počela je kiša sounds smooth and natural after the first clause. It also puts the verb first, which can make the event feel a bit more immediate or narrative.

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

In Croatian perfect tense, the participle agrees with the subject in:

• gender
• number

So:

• kiša = feminine singular
• therefore počela

Compare:

• Počeo je film. = The film started.
  (film is masculine singular)
• Počela je kiša. = The rain started.
  (kiša is feminine singular)
• Počela je utakmica. = The match started.

Because Croatian does not need a dummy subject like English it.

In English, we say:

• It is raining
• It started raining

But that it does not really refer to anything. It is just required by English grammar.

Croatian usually does not use this kind of empty subject. Instead, it says something like:

• Pada kiša. = It is raining. literally Rain is falling.
• Počela je kiša. = It started raining. literally The rain started.

So the Croatian sentence uses kiša as the real subject.

Kući is a very common special form meaning home or to home / at home, depending on context.

In this sentence:

• stigli kući = arrived home

This is idiomatic Croatian. With home, Croatian often uses kući without a preposition.

Compare:

• Idem kući. = I’m going home.
• Stigli smo kući. = We arrived home.
• Kod kuće sam. = I am at home.

So even though English uses home without a preposition too, the Croatian form is specifically kući.

A learner should mostly memorize this as a set phrase:

• ići kući = to go home
• doći kući = to come home
• stići kući = to arrive home

Yes, Samo što smo došli kući, počela je kiša is also natural.

The difference is small:

• doći = to come
• stići = to arrive / reach

In many contexts, both work when talking about getting home.

Nuance:

• došli kući focuses a bit more on coming
• stigli kući focuses a bit more on arriving/reaching the destination

In everyday speech, both are common and both sound normal in this sentence.

Because both actions are presented as completed events.

The verbs are:

• stići = perfective, to arrive
• početi = perfective, to begin/start

This fits the meaning very well:

1. we arrived
2. then the rain started

Each event is seen as a single completed whole.

If you used imperfective verbs, the meaning would change or sound less natural here. Croatian often uses perfective verbs in past narration when one event happens and then another begins right after it.

Yes, the comma is standard here.

The first part:

• Samo što smo stigli kući

is a subordinate clause-like expression giving the time relation, and the second part:

• počela je kiša

is the main clause.

So the comma helps separate:

• the just after/as soon as clause
• the main event

That is why the normal punctuation is:

• Samo što smo stigli kući, počela je kiša.

Yes, often you can.

For example:

• Čim smo stigli kući, počela je kiša.

This means roughly:

• As soon as we got home, it started raining.

The difference is nuance:

• čim = as soon as
• samo što = just after / no sooner had... than

Samo što often gives a stronger feeling that the second event happened almost immediately, with barely any gap at all.

So both are correct, but samo što is especially good when you want to stress how quickly the rain started.

Yes, and this can confuse learners.

Besides the meaning it has here, samo što can also appear in other contexts with meanings closer to:

• except that
• only that
• the only thing is that

So you should not translate it word-for-word every time. You need to look at the whole sentence.

In your example:

• Samo što smo stigli kući, počela je kiša.

it clearly means just after / as soon as.

So this is a good expression to learn as a chunk.

Because the participle in Croatian agrees with the subject.

The subject is we, understood from smo.

For we in the past tense:

• stigli = masculine plural or mixed group
• stigle = feminine plural

So:

• Samo što smo stigli kući...
  = used if the speakers are male or mixed, or if gender is unspecified
• Samo što smo stigle kući...
  = used if the speakers are all female

This is something English does not show, but Croatian does.

Yes. Croatian allows some flexibility.

For example, you could also hear:

• Kiša je počela samo što smo stigli kući.
• Samo što smo stigli kući, kiša je počela.

These are all understandable, but the original:

• Samo što smo stigli kući, počela je kiša.

sounds very natural and idiomatic.

The original order is especially good for storytelling because it first sets up the moment, then gives the sudden event.