Tek što smo upalili roštilj, počela je kiša i svi su potrčali prema autu.

Here tek što means just after, no sooner than, or just as.

So:

• Tek što smo upalili roštilj...
  = We had just lit the grill... = Just after we lit the grill... = No sooner had we lit the grill...

It gives a strong sense that the next event happened almost immediately.

In this sentence, it emphasizes the bad timing: as soon as the grill was lit, it started raining.

Both word orders can exist in Croatian, but smo is a clitic (a short unstressed word), and clitics usually go in the second position of the clause.

So in:

• Tek što smo upalili roštilj

the opening unit tek što counts as the first element, and then the clitic smo comes right after it.

That is why smo appears before upalili here.

This is very typical Croatian word order:

• Tek što smo došli...
• Kad je stigao...
• Odmah su otišli...

Clitics like sam, si, je, smo, ste, su often appear very early in the sentence or clause.

It is the Croatian perfect tense, which is the normal past tense in everyday Croatian.

It is formed with:

• the present tense of biti (sam, si, je, smo, ste, su)
• plus the past participle

So:

• smo upalili = we lit / we have lit
• je počela = started / has started
• su potrčali = ran off / started running

In natural English, you usually translate these simply as the past tense:

• we lit
• it started raining
• they ran

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

• upaliti = to switch on / light, perfective
• paliti = to light / be lighting, imperfective

Here the speaker is talking about a completed action: the grill was lit, and immediately after that it started raining.

So upalili is the natural choice because it presents the action as a single finished event:

• Tek što smo upalili roštilj...
  = We had just lit the grill...

If you used palili, it would sound more like an ongoing or repeated action, which does not fit as well here.

Yes, literally upaliti often means to switch on or to light, depending on the object.

With roštilj, it means to light the grill or start the grill.

Examples:

• upaliti svjetlo = turn on the light
• upaliti auto = start the car
• upaliti cigaretu = light a cigarette
• upaliti roštilj = light/start the grill

So the same verb is used in several related situations.

Because roštilj is the direct object of upalili, so it is in the accusative case.

However, roštilj is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: roštilj
• accusative: roštilj

That is why the form does not change.

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

Again, je is a clitic, and clitics tend to come early. Croatian word order is flexible, and speakers often arrange the sentence to sound natural, rhythmic, or slightly emphasize one part.

So these are both possible:

• Počela je kiša.
• Kiša je počela.

The first one is often more natural in narration.

Also, Croatian does not need a dummy subject like English it in it started raining. Instead, it simply says:

• Počela je kiša
  literally: Rain started

Croatian can express this idea in more than one way.

Here, the sentence uses:

• počela je kiša = the rain started

This is a very common and natural way to say it started raining.

Croatian can also use the verb padati:

• Počela je padati kiša.
• Kiša je počela padati.

These also mean It started raining.

The shorter version počela je kiša is very idiomatic and common.

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

In the perfect tense, the past participle agrees in gender and number with the subject.

So:

• kiša = feminine singular
• therefore: počela

Compare:

• Vjetar je počeo. = The wind started.
  (vjetar is masculine singular)
• Oluje su počele. = The storms started.
  (oluje is feminine plural)

This is another aspect question.

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

In this sentence:

• svi su potrčali prema autu

means that everyone suddenly broke into a run or started running toward the car.

If you said trčali su, it would focus more on the ongoing action they were running.
But potrčali su fits much better here because it describes the sudden reaction to the rain.

Prema takes the dative case, so auto becomes autu.

• nominative: auto
• dative: autu

So:

• prema autu = toward the car

This is a very important pattern:

• prema kući = toward the house
• prema gradu = toward the city
• prema meni = toward me

Yes. Auto and automobil both mean car.

So you could say:

• prema autu
• prema automobilu

The first is more everyday and conversational. The second is a bit more formal or neutral.

Because Croatian often drops subject pronouns when they are not needed.

The verb form already shows the person and number:

• smo upalili = we lit
• su potrčali = they ran / everyone ran

Also, in the second clause, the subject is explicitly given as svi = everyone / all.

So Croatian does not need:

• mi smo upalili
• oni su potrčali

unless the speaker wants extra emphasis or contrast.

Svi means everyone or all of them.

In this sentence:

• svi su potrčali prema autu

it means that all the people present ran toward the car.

Grammatically, svi is plural, so the verb is also plural:

• svi su potrčali

Yes, basically yes. The sentence is telling a sequence of quick, finished events, and that is why perfective verbs work so well here:

• upalili from upaliti — completed act of lighting the grill
• počela from početi — the rain began
• potrčali from potrčati — everyone suddenly started running

This gives the sentence a very dynamic feel: one event happened, then immediately another, then a quick reaction.

It is very natural. It sounds like normal spoken or written Croatian narrative.

A native speaker would immediately understand it as:

• We had just lit the grill,
• then it started raining,
• and everyone ran toward the car.

It is a good example of several common Croatian features at once:

• tek što for immediate succession
• perfect tense
• perfective verbs
• flexible word order
• clitic placement
• dative after prema