Ο κεραμικός κλίβανος λειτουργεί σήμερα.

A word-by-word breakdown is:

• Ο = the (masculine singular nominative article)
• κεραμικός = ceramic, pottery-related
• κλίβανος = kiln
• λειτουργεί = operates / is working / is functioning
• σήμερα = today

So the structure is basically:

• The ceramic kiln
  • operates / is functioning
    • today

Ο is the masculine singular form of the definite article the.

It matches κλίβανος, which is:

• masculine
• singular
• nominative (the subject form)

Because κεραμικός κλίβανος means the ceramic kiln and the noun is the subject of the sentence, Greek uses ο.

Compare:

• ο κλίβανος = the kiln
• ένας κλίβανος = a kiln

Because the adjective must agree with the noun it describes.

The noun κλίβανος is:

• masculine
• singular
• nominative

So the adjective also appears in the:

• masculine
• singular
• nominative form

That gives:

• κεραμικός for masculine
• κεραμική for feminine
• κεραμικό for neuter

Examples:

• ο κεραμικός κλίβανος = the ceramic kiln
• η κεραμική τέχνη = ceramic art
• το κεραμικό υλικό = the ceramic material

It is in the nominative case, because it is the subject of the sentence.

You can tell from the article and endings:

• ο is the masculine singular nominative article
• κεραμικός has the nominative masculine singular ending -ός
• κλίβανος has the nominative singular form

So Greek is marking the ceramic kiln as the thing doing the action: it is the thing that λειτουργεί.

It can suggest either, depending on context.

κεραμικός can mean:

• made of ceramic
• related to ceramics
• used in pottery/ceramic work

With κλίβανος, the most natural meaning is a kiln used for ceramics/pottery.

So although κεραμικός often literally means ceramic, in this phrase it usually means something like:

• ceramic kiln
• pottery kiln
• kiln for ceramics

Here λειτουργεί means:

• is operating
• is working
• is functioning
• is in operation

The verb λειτουργώ often means that a machine, service, business, or institution is operating properly or open and running.

So in this sentence, it does not mean the kiln is merely existing; it means it is working / running today.

Because λειτουργεί already contains the meaning he/she/it operates.

Greek verbs usually include the subject information in the verb ending, so you often do not need an extra word like English is.

• λειτουργώ = I operate / function
• λειτουργείς = you operate / function
• λειτουργεί = he/she/it operates / functions

So λειτουργεί by itself already means it operates / it is functioning.

It is the present tense, third person singular, from λειτουργώ.

That is why it means:

• operates
• is operating
• works
• is working
• functions
• is functioning

Greek present tense often covers both the simple present and the present progressive, depending on context.

So λειτουργεί σήμερα can naturally mean:

• operates today
• is operating today
• is working today

A rough pronunciation guide is:

• Ο = o
• κεραμικός = keh-ra-mee-KOS
• κλίβανος = KLEE-va-nos
• λειτουργεί = lee-tour-GHEE
• σήμερα = SEE-me-ra

Roughly: o keh-ra-mee-KOS KLEE-va-nos lee-tour-GHEE SEE-me-ra

A few stress points:

• κεραμικός is stressed on the last syllable: -κός
• κλίβανος is stressed on the first syllable: κλί-
• λειτουργεί is stressed on the last syllable: -γεί
• σήμερα is stressed on the first syllable: σή-

Yes, it can move.

Greek word order is more flexible than English because the endings show grammatical roles more clearly. The version you have is a very natural neutral order:

• Ο κεραμικός κλίβανος λειτουργεί σήμερα.

But you could also say:

• Σήμερα λειτουργεί ο κεραμικός κλίβανος.
• Ο κεραμικός κλίβανος σήμερα λειτουργεί.

These all mean roughly the same thing, but the emphasis changes a little:

• final σήμερα sounds neutral and straightforward
• initial Σήμερα emphasizes today
• putting σήμερα in the middle can sound slightly more marked or stylistic

In a normal full sentence, keeping the article is the most natural choice:

• Ο κεραμικός κλίβανος λειτουργεί σήμερα.

If you remove ο, the sentence sounds less like ordinary standard prose and more like:

• a label
• a heading
• a note
• a very compressed style

Greek uses the definite article more often than English, so learners should usually include it when talking about a specific noun like this.

Not usually in everyday household contexts.

κλίβανος is a more specialized word, often meaning:

• kiln
• furnace
• an industrial or technical heating chamber

For an ordinary oven, Greek more often uses:

• φούρνος

So:

• κλίβανος = kiln / furnace / industrial oven
• φούρνος = oven / bakery / stove oven, depending on context

That is why κλίβανος fits very well here.

Yes, it can.

Adding σήμερα often highlights that today is relevant. Depending on context, the sentence might suggest:

• it is operating today in particular
• it is open/running today
• today is one of the days it is active
• perhaps on other days it is not operating

The sentence by itself does not force that interpretation, but it often carries that practical sense.

Yes. In Greek, an adjective can appear before or after the noun, but before the noun is very common and natural here:

• ο κεραμικός κλίβανος

That is a standard noun phrase: article + adjective + noun.

You may also see adjectives after the noun in Greek, but that can sound different in rhythm, emphasis, or style depending on the phrase.

For this sentence, ο κεραμικός κλίβανος is the most straightforward choice.

The most literal translation is:

• The ceramic kiln operates today.

More natural English translations could be:

• The ceramic kiln is operating today.
• The pottery kiln is working today.
• The kiln is in operation today.

Which one sounds best depends on context.