Τρομάζω όταν χτυπάει ξαφνικά το θυροτηλέφωνο τη νύχτα.

Because Greek often leaves out subject pronouns when the verb already shows who the subject is.

Here, τρομάζω means I get scared / I get startled, so εγώ is not necessary.
If you added εγώ, it would usually sound emphatic, like:

Εγώ τρομάζω... = I get scared...

So the version without εγώ is the most natural neutral wording.

The present tense here describes a habitual or general reaction, not something happening only once.

So the sentence means something like:

• I get startled when...
• I get scared whenever...

It is not mainly about one specific night. It is about what typically happens to the speaker.

If you wanted a one-time past event, you would use a past form, for example: Τρόμαξα όταν... = I got scared when...

It can mean both, depending on context. In this sentence, because both verbs are in the present and the idea is habitual, όταν is best understood as when/whenever.

So:

Τρομάζω όταν χτυπάει...
means
I get startled when / whenever it rings...

If the sentence were about one specific event, όταν would simply mean when.

For the same reason: the sentence describes something that happens generally or repeatedly.

Greek often uses the present tense in both clauses for this kind of meaning:

• Τρομάζω = I get startled
• όταν χτυπάει = when/whenever it rings

This is very natural Greek for repeated situations.

Compare:

• Τρομάζω όταν χτυπάει το θυροτηλέφωνο.
  = I get startled when the intercom rings.
  habitual / general

• Τρόμαξα όταν χτύπησε το θυροτηλέφωνο.
  = I got startled when the intercom rang.
  one past event

Both are correct.

This verb can appear in two common present-tense forms:

• χτυπάει
• χτυπά

So:

• όταν χτυπάει το θυροτηλέφωνο
• όταν χτυπά το θυροτηλέφωνο

both mean the same thing.

χτυπάει may sound a little fuller or more colloquial in some contexts, while χτυπά is also very common. A learner should recognize both.

Yes, the basic meaning of χτυπάω / χτυπώ is something like hit, strike, or knock. But in Greek it is also very commonly used for sounds made by devices:

• χτυπάει το τηλέφωνο = the phone is ringing
• χτυπάει το κουδούνι = the bell is ringing
• χτυπάει το θυροτηλέφωνο = the intercom/doorbell is ringing or buzzing

So this is a normal everyday extension of the verb, just like English uses ring for phones and bells.

Greek word order is more flexible than English word order.

English strongly prefers:

• the intercom rings

Greek can say:

• χτυπάει το θυροτηλέφωνο
• το θυροτηλέφωνο χτυπάει

Both are possible, but they do not always sound equally natural in every context.

In this sentence, χτυπάει το θυροτηλέφωνο sounds very natural because the focus is on the event happening: the intercom suddenly rings. Putting the verb first is common in Greek, especially when introducing an action or event.

Ξαφνικά means suddenly, and here it modifies the action χτυπάει.

So the idea is:

• the intercom rings suddenly
• it rings all of a sudden

It does not describe the speaker becoming suddenly scared; it describes the ringing as sudden.

Its position is flexible. Greek could also say:

• όταν το θυροτηλέφωνο χτυπάει ξαφνικά
• όταν ξαφνικά χτυπάει το θυροτηλέφωνο

The sentence you were given is a very natural placement.

Because τη νύχτα is a time expression in the accusative, used adverbially to mean at night or during the night.

Greek often uses the accusative without a preposition for expressions of time:

• τη νύχτα = at night
• την Κυριακή = on Sunday
• το καλοκαίρι = in summer

So τη νύχτα does not mean the night as a direct object here. It functions like an adverbial phrase of time.

Because the final -ν of την is often dropped before certain consonants.

So:

• τη νύχτα is the normal form here

The full form την is kept more often before vowels or certain consonants, but before ν in νύχτα, dropping the final -ν is standard and very common.

A learner should recognize that:

• τη = shortened form of την
• both come from the feminine accusative article

Not with exactly the same meaning.

• τρομάζω = I get startled / I get frightened suddenly
• φοβάμαι = I am afraid / I feel fear

In this sentence, the idea is a sudden reaction to an unexpected sound at night, so τρομάζω is the better choice.

Compare:

• Τρομάζω όταν χτυπάει ξαφνικά το θυροτηλέφωνο τη νύχτα.
  = I get startled when the intercom suddenly rings at night.

• Φοβάμαι όταν χτυπάει το θυροτηλέφωνο τη νύχτα.
  = I’m afraid when the intercom rings at night.

The second sentence suggests a more general feeling of fear, not just a sudden jolt.

It most naturally describes a general situation.

The present tense in both clauses gives it the sense of:

• This is what happens to me
• This is how I react in that kind of situation

If you wanted to describe one specific event, Greek would normally change the verb forms, for example:

• Τρόμαξα όταν χτύπησε ξαφνικά το θυροτηλέφωνο τη νύχτα.
  = I got startled when the intercom suddenly rang at night.

So the original sentence is best understood as habitual or repeated.