Ο ανιχνευτής καπνού λειτουργεί.

Ο is the masculine singular definite article, meaning the.

In Ο ανιχνευτής καπνού λειτουργεί, ο goes with ανιχνευτής because ανιχνευτής is a masculine singular noun.

So:

• ο = the
• ανιχνευτής = detector

Together: ο ανιχνευτής = the detector

Ανιχνευτής means detector.

It comes from a verb related to detecting / tracing / identifying something. In everyday usage, ανιχνευτής is used for devices that detect something, such as:

• ανιχνευτής καπνού = smoke detector
• ανιχνευτής μετάλλων = metal detector

In this sentence, it is the subject of the verb.

Because Greek is using the genitive case here.

• καπνός = smoke as a basic dictionary form
• καπνού = of smoke

So ανιχνευτής καπνού literally means detector of smoke, which is how Greek normally says smoke detector.

This is very common in Greek. English often uses noun + noun compounds, but Greek often uses:

• noun + genitive noun

Examples:

• ποτήρι νερού = glass of water
• μηχανή καφέ = coffee machine
• ανιχνευτής καπνού = smoke detector

Here is the grammatical breakdown:

• Ο — definite article, masculine singular nominative
• ανιχνευτής — noun, masculine singular nominative
• καπνού — noun, masculine singular genitive
• λειτουργεί — verb, 3rd person singular present

So:

• ο ανιχνευτής is the subject
• καπνού depends on ανιχνευτής and means of smoke
• λειτουργεί is the verb: works / functions / operates

Λειτουργεί means works, is working, functions, or operates.

In this sentence, the most natural English meaning is something like:

• The smoke detector works
• The smoke detector is working
• The smoke detector is functioning

Greek present tense can sometimes correspond to either a simple present or a present progressive in English, depending on context.

The dictionary form is usually given as λειτουργώ.

That means:

• λειτουργώ = I work / I function / I operate
• λειτουργεί = he/she/it works or functions

Here the subject is the smoke detector, so Greek uses the 3rd person singular form:

• ο ανιχνευτής καπνού λειτουργεί

A helpful approximate pronunciation is:

o ani-hnev-TIS kap-NOO lee-toor-GHEE

A few notes:

• χ sounds like the ch in Scottish loch or German Bach
• η, ι, υ, ει, οι often sound like ee in Modern Greek
• ου sounds like oo
• Stress matters a lot in Greek:
  • ανιχνευτής
  • καπνού
  • λειτουργεί

Yes. Greek word order is more flexible than English word order.

The neutral, straightforward order is:

• Ο ανιχνευτής καπνού λειτουργεί.

But you could also hear:

• Λειτουργεί ο ανιχνευτής καπνού.

That might sound more like The smoke detector is working with slight emphasis on the action or on the fact that it works.

Still, the original order is the most basic and learner-friendly version.

It can mean either one, depending on context.

Greek present tense often covers both ideas:

• general truth / normal function
  The smoke detector works
• current situation
  The smoke detector is working

If someone has just tested it, λειτουργεί could mean it’s working now.
If someone is describing the device, it could mean it works / it functions in general.

Context tells you which meaning is intended.

Because λειτουργεί is already a complete verb meaning works / functions / operates.

In English, you might say:

• The smoke detector is working

But is working is really just the verb work in a progressive form. Greek simply uses the present tense verb λειτουργεί.

So there is no missing word here. The sentence is complete as it is.

You add δεν before the verb:

• Ο ανιχνευτής καπνού δεν λειτουργεί.

That means:

• The smoke detector is not working
• The smoke detector doesn’t work

This is the normal way to negate a present-tense verb in Greek.

Yes, it is a normal and correct way to say it.

Literally, it means detector of smoke, but that is exactly how Greek commonly expresses this kind of idea.

A native speaker will understand it naturally. It is a good phrase to learn as a set expression:

• ανιχνευτής καπνού = smoke detector

Only because it is the first word of the sentence.

Greek capitalization works much like English capitalization here:

• At the start of a sentence: Ο ανιχνευτής καπνού λειτουργεί.
• In the middle of a sentence: Ο τεχνικός είπε ότι ο ανιχνευτής καπνού λειτουργεί.

So ο is not normally capitalized by itself; it is capitalized here only because it starts the sentence.