Η κόρνα του αυτοκινήτου ακούστηκε ξαφνικά, και το παιδί τρόμαξε.

Because Greek uses the genitive case to show possession.

• η κόρνα = the horn
• του αυτοκινήτου = of the car

So η κόρνα του αυτοκινήτου literally means the horn of the car, which English usually expresses as the car's horn or the car horn.

Here:

• του is the genitive singular form of the
• αυτοκινήτου is the genitive singular of αυτοκίνητο

Here are the main noun phrases:

• η κόρνα

  • feminine
  • nominative singular
  • the subject of ακούστηκε

• του αυτοκινήτου

  • neuter
  • genitive singular
  • shows possession: of the car

• το παιδί

  • neuter
  • nominative singular
  • the subject of τρόμαξε

A useful thing to remember: Greek noun gender is grammatical, not always biological. So παιδί is grammatically neuter, even if the child is a boy or a girl.

Both are in the aorist, which is the normal Greek tense for a single completed past event.

So the sentence describes two events that happened and are finished:

• the horn sounded / was heard
• the child got scared

The aorist is often the best match for English simple past in storytelling.

Ακούστηκε comes from ακούγομαι, which means to be heard or sometimes to sound.

So literally, η κόρνα του αυτοκινήτου ακούστηκε is something like:

• the car horn was heard

But in natural English, that is often translated more smoothly as:

• the car horn sounded
• the sound of the car horn was heard

So yes, the form is passive-looking, but in this kind of sentence it works very naturally to describe a sound becoming audible.

If you wanted to say someone heard the horn, Greek would use a different structure, for example:

• Άκουσε την κόρνα = He/She heard the horn

The dictionary form is τρομάζω.

In the aorist, many Greek verbs change form, and τρομάζω becomes:

• τρόμαξα = I got scared / I frightened
• τρόμαξε = he/she/it got scared

So the -ζ- changes to -ξ- in the aorist stem. That is a normal pattern in Greek and something learners get used to over time.

In this sentence, το παιδί τρόμαξε means:

• the child got scared
• the child was frightened

Here it means got scared.

The verb τρομάζω can sometimes be used:

• transitively: frighten someone
• intransitively: get scared

In this sentence there is no object after the verb, so το παιδί τρόμαξε is understood as:

• the child got frightened
• the child was startled

The context makes that clear.

Ξαφνικά means suddenly.

It functions as an adverb, and here it modifies the event ακούστηκε. In other words, it tells you how the horn was heard/sounded: it happened suddenly.

So:

• ακούστηκε ξαφνικά = was heard suddenly / sounded suddenly

It can also move around in the sentence without changing the basic meaning very much, for example:

• Ξαφνικά ακούστηκε η κόρνα του αυτοκινήτου
• Η κόρνα του αυτοκινήτου ακούστηκε ξαφνικά

The difference is mostly one of emphasis.

Because και here joins two full clauses, each with its own verb:

• Η κόρνα του αυτοκινήτου ακούστηκε ξαφνικά
• το παιδί τρόμαξε

In Greek, it is common to use a comma before και when two complete clauses are being linked, especially in narrative writing. English can be stricter about commas before and, but Greek punctuation is often a bit more flexible in this area.

So the comma here is natural and helps separate the two events.

Yes. Greek word order is more flexible than English because case endings and articles help show what each word is doing.

For example, these are all possible with slightly different emphasis:

• Η κόρνα του αυτοκινήτου ακούστηκε ξαφνικά, και το παιδί τρόμαξε.
• Ξαφνικά ακούστηκε η κόρνα του αυτοκινήτου, και το παιδί τρόμαξε.
• Η κόρνα του αυτοκινήτου ακούστηκε ξαφνικά, και τρόμαξε το παιδί.

The original version is a very neutral, clear way to say it.

A rough pronunciation guide is:

i KOR-na tu af-to-ki-NI-tu a-KU-sti-ke, ksef-ni-KA, ke to pe-THEE TRO-mak-se

A few helpful sound notes:

• κόρνα → stress on KOR
• αυτοκινήτου → stress on NI
• ακούστηκε → stress on KU
• ξαφνικά → stress on KA
• παιδί → stress on the last syllable, THEE
• τρόμαξε → stress on TRO

And some letter combinations:

• αι usually sounds like e
• ου sounds like oo
• ξ sounds like ks
• δ in παιδί sounds like th in this

The accent marks show which syllable is stressed, and stress is very important in Greek pronunciation.