Mali vrabac skočio je do stola čim je vidio mrvice, a djevojčica ga je samo gledala.

It uses the past tense, specifically the Croatian perfect.

You can see it in:

• skočio je = he jumped
• je vidio = he saw
• je gledala = she watched / was watching

This tense is formed with:

• the auxiliary biti in the present tense, here je
• plus the past participle: skočio, vidio, gledala

So the pattern is basically:

• je + participle or participle + je, depending on word order.

Because mali is an adjective and it must agree with vrabac in:

• gender
• number
• case

Here:

• vrabac = masculine singular nominative
• so the adjective is also masculine singular nominative: mali

Compare:

• mali vrabac = a small sparrow
• mala djevojčica = a small girl
• malo dijete = a small child

Because the subject of that clause is vrabac, which is masculine singular.

In Croatian, the past participle agrees with the subject:

• on je vidio = he saw
• ona je vidjela = she saw
• ono je vidjelo = it saw

So in čim je vidio mrvice, the hidden subject is still the sparrow, so vidio is masculine.

Because the subject of the second clause is djevojčica, and djevojčica is feminine singular.

So:

• djevojčica je gledala = the girl watched

Again, the participle agrees with the subject:

• gledao = masculine
• gledala = feminine
• gledalo = neuter

Croatian word order is much more flexible than English word order.

A few important points:

• je is an unstressed word, called a clitic
• ga is also an unstressed clitic
• clitics usually appear near the beginning of their clause, but not in just any position

So:

• skočio je is a normal past-tense order
• ga je is also a normal clitic sequence in this sentence

A more neutral version could also be:

• Mali vrabac je skočio do stola...

The given version, Mali vrabac skočio je..., sounds natural too, and gives a slightly more narrative or stylistic rhythm.

Čim means as soon as or once.

So:

• čim je vidio mrvice = as soon as he saw the crumbs

It introduces a time clause.

Examples:

• Čim dođe, nazovi me. = As soon as he/she arrives, call me.
• Čim sam ga vidio, znao sam. = As soon as I saw him, I knew.

Stola is in the genitive singular because the preposition do takes the genitive.

So:

• do stola = to/up to the table, as far as the table

Important nuance:

• do stola usually means up to / near the table
• na stol would mean onto the table

So this phrase does not necessarily mean that the sparrow landed on top of the table. It means he jumped to the table / up to it.

Mrvice is the accusative plural of mrvica (crumb).

Here it is the direct object of vidio:

• vidio mrvice = he saw crumbs

For this noun, the nominative plural and accusative plural have the same form:

• mrvice = crumbs

So the case is understood from its role in the sentence, not from a different ending.

Ga refers to vrabac.

It means him/it, but because vrabac is a masculine animate noun, Croatian uses the masculine accusative clitic ga.

So:

• djevojčica ga je samo gledala = the girl only watched him

Here ga is the direct object of gledala.

A often connects two clauses with a mild contrast, shift of focus, or parallel action.

Here it works a bit like:

• and
• while
• whereas

So:

• ..., a djevojčica ga je samo gledala. means something like:
• ..., and meanwhile the girl just watched him
• ..., while the girl only watched him

Compare:

• i = simple and
• ali = but, stronger contrast
• a = softer contrast or transition

Samo means only or just.

In this sentence:

• djevojčica ga je samo gledala

it suggests that the girl did nothing else. She did not chase him away, catch him, or help him; she just watched.

So samo adds emphasis.

Because Croatian normally has no articles.

English says:

• the sparrow
• a girl
• the crumbs

Croatian usually just says:

• vrabac
• djevojčica
• mrvice

Whether something is definite or indefinite is understood from:

• context
• word order
• the situation
• sometimes extra words like taj (that) or jedan (one/a certain)

So Croatian does not need separate words exactly like English the and a.

Yes, aspect is part of what makes this sentence feel natural.

• skočiti is perfective: a completed single action
  
  • skočio je = he jumped
• vidjeti is usually treated as perfective in this kind of event meaning
  
  • čim je vidio = as soon as he saw / caught sight of
• gledati is imperfective: an ongoing action
  
  • gledala je = she was watching / watched

So the sentence contrasts:

• the sparrow’s quick completed action with
• the girl’s continuing observation

That is a very common Croatian storytelling pattern.

A rough guide:

• čim sounds approximately like cheem
• djevojčica is roughly dye-voich-ee-tsa

A few useful letter notes:

• č = like ch in church, but firmer
• c = like ts
• dj / đ-type sound in dje... is softer than plain English j

You do not need a perfect accent right away; the main thing is to notice:

• č is not the same as c
• c in Croatian is ts, never English k or s by itself.