Ο σκύλος γαβγίζει συνέχεια όταν ακούει το θυροτηλέφωνο.

A word-by-word breakdown is:

• Ο = the
• σκύλος = dog
• γαβγίζει = barks / is barking
• συνέχεια = continuously / all the time / constantly
• όταν = when / whenever
• ακούει = hears
• το = the
• θυροτηλέφωνο = intercom / doorphone

So the sentence structure is basically:

The dog barks constantly when it hears the intercom.

Ο and το are the definite articles, meaning the.

In this sentence:

• ο σκύλος = the dog
• το θυροτηλέφωνο = the intercom

Greek uses articles very regularly, often more regularly than English. With a normal singular noun like dog or intercom, you usually do need the article if you mean a specific one.

Also, the article shows gender, number, and case, so it gives extra grammar information, not just definiteness.

Because σκύλος is:

• masculine
• singular
• nominative case

So its article is ο.

This noun is the subject of the sentence, so it appears in the nominative:

• ο σκύλος = the dog as the one doing the action

If the noun changed role, its form could change too. For example, as a direct object you would get τον σκύλο.

Good question. Το θυροτηλέφωνο is the direct object of ακούει, so it is in the accusative case.

But θυροτηλέφωνο is a neuter noun, and in the singular, many neuter nouns have the same form in the nominative and accusative.

So:

• nominative: το θυροτηλέφωνο
• accusative: το θυροτηλέφωνο

The case changes grammatically, but the visible form happens to stay the same.

Both are 3rd person singular present tense forms.

• γαβγίζει comes from γαβγίζω = to bark
• ακούει comes from ακούω = to hear

So:

• γαβγίζει = it barks / he/she/it is barking
• ακούει = it hears / he/she/it is hearing

In this sentence, both refer to the dog, even though Greek does not repeat the subject.

Because Greek is a pro-drop language. That means subject pronouns are often omitted when they are clear from the verb form and the context.

So Greek normally says:

• ακούει = it hears / he hears / she hears

The ending -ει already tells you it is 3rd person singular, and the context tells you the subject is the dog.

English usually needs it, but Greek often does not.

Here συνέχεια is being used adverbially, meaning:

• constantly
• continuously
• all the time

So γαβγίζει συνέχεια means something like:

• barks constantly
• keeps barking

Even though συνέχεια is also a noun in other contexts, here it functions like an adverb, so there is no article.

You can think of it as similar to saying non-stop.

Όταν means when or whenever.

In this sentence it introduces a time clause:

• όταν ακούει το θυροτηλέφωνο = when / whenever it hears the intercom

Because this is a habitual situation, όταν can feel closer to whenever in English:

• The dog barks constantly whenever it hears the intercom.

Greek often uses the present tense for actions that happen regularly or habitually.

So this sentence is not talking about just one single moment. It describes a repeated pattern:

• The dog barks constantly whenever it hears the intercom.

That is why Greek uses:

• γαβγίζει
• ακούει

both in the present.

This is very natural in Greek and corresponds well to English habitual present.

No. Ακούω normally takes a direct object.

So you say:

• ακούω μουσική = I listen to music / I hear music
• ακούω το ραδιόφωνο = I listen to the radio
• ακούει το θυροτηλέφωνο = it hears the intercom

So there is no preposition before το θυροτηλέφωνο.

This is often easier than English, because English has both hear and listen to, while Greek often uses ακούω for both kinds of idea depending on context.

Θυροτηλέφωνο is a compound word meaning doorphone / intercom.

It is built from:

• θύρα = door
• τηλέφωνο = telephone

In everyday English, depending on the situation, you might translate it as:

• intercom
• doorphone
• sometimes even buzzer, if that is what is meant in context

It is a neuter noun, which is why the article is το.

Yes. Greek word order is more flexible than English.

The original sentence is a very natural, neutral version:

• Ο σκύλος γαβγίζει συνέχεια όταν ακούει το θυροτηλέφωνο.

But Greek could also say, for example:

• Όταν ακούει το θυροτηλέφωνο, ο σκύλος γαβγίζει συνέχεια.
• Ο σκύλος συνέχεια γαβγίζει όταν ακούει το θυροτηλέφωνο.

These versions are still understandable, but the focus or emphasis may shift slightly.

The original order is a good standard model for learners.

A rough pronunciation guide is:

• Ο σκύλος → o SKEE-los
• γαβγίζει → ghav-YEE-zee
• συνέχεια → see-NE-chee-a
• όταν → O-tan
• ακούει → a-KOO-ee
• το θυροτηλέφωνο → to thee-ro-tee-LE-fo-no

A full rough guide:

o SKEE-los ghav-YEE-zee see-NE-chee-a O-tan a-KOO-ee to thee-ro-tee-LE-fo-no

Also, the written accent marks in Greek show you where the stress goes:

• σκύλος
• γαβγίζει
• συνέχεια
• όταν
• ακούει
• θυροτηλέφωνο