Το κλιματιστικό δεν λειτουργεί καλά σήμερα.

Το is the definite article here, meaning the.

In this sentence, το κλιματιστικό means the air conditioner.

A few useful points:

• το is used with neuter singular nouns in the nominative or accusative
• κλιματιστικό is a neuter noun
• Greek articles change much more than English the

So:

• το κλιματιστικό = the air conditioner

κλιματιστικό is a neuter singular noun. In this sentence, it is the subject, so it is in the nominative form.

The basic dictionary form is το κλιματιστικό.

This word literally comes from the idea of air conditioning, and in everyday Greek it commonly means:

• air conditioner
• sometimes more generally air-conditioning unit

Because it is neuter, it goes with:

• το in the singular
• τα in the plural

For example:

• το κλιματιστικό
• τα κλιματιστικά

δεν is the normal way to make a verb negative in Modern Greek. It means not.

So:

• λειτουργεί = works / is working / functions
• δεν λειτουργεί = does not work / is not working / is not functioning

This is very common in Greek:

• δεν ξέρω = I don’t know
• δεν θέλω = I don’t want
• δεν βλέπω = I don’t see

A useful thing to remember: δεν usually comes directly before the verb.

λειτουργεί is the 3rd person singular form of the verb λειτουργώ, which means to function / to work / to operate.

Since the subject is the air conditioner, Greek uses he/she/it works = λειτουργεί.

Very roughly:

• λειτουργώ = I function / I work
• λειτουργείς = you function / you work
• λειτουργεί = he/she/it functions / works

Greek verbs change according to the subject, so you do not need a separate word for it.

Greek usually does not need subject pronouns when the verb ending already shows the person.

In English, you must say:

• It is not working

In Greek, the verb form itself already tells you the subject is 3rd person singular, and the noun το κλιματιστικό is already stated, so there is no need for a separate pronoun.

That is why Greek naturally says:

• Το κλιματιστικό δεν λειτουργεί καλά σήμερα.

and not something with an extra it.

Because καλά is an adverb here, not an adjective.

It modifies the verb λειτουργεί and means:

• well
• properly

So:

• λειτουργεί καλά = works well / works properly

Compare:

• καλός / καλή / καλό = good as an adjective, describing a noun
• καλά = well as an adverb, describing how something works, goes, etc.

Examples:

• Είναι καλό κλιματιστικό. = It is a good air conditioner.
• Το κλιματιστικό λειτουργεί καλά. = The air conditioner works well.

λειτουργεί καλά means works well, is working properly, or functions well.

With machines or devices, λειτουργώ is very common. So this phrase can suggest:

• it is not operating properly
• it is having problems
• it is underperforming

Because the sentence says δεν λειτουργεί καλά, the idea is not necessarily that it is completely broken. It may simply be:

• not working very well
• not working properly today

So this is slightly softer than saying it is totally out of order.

σήμερα means today.

It tells you when the problem is happening:

• Το κλιματιστικό δεν λειτουργεί καλά σήμερα.
• The air conditioner isn’t working well today.

Yes, Greek word order is fairly flexible, so σήμερα can move depending on emphasis. For example:

• Σήμερα το κλιματιστικό δεν λειτουργεί καλά.
• Το κλιματιστικό σήμερα δεν λειτουργεί καλά.

These are all natural, though the original version is very straightforward and neutral.

It can be either, depending on context.

Greek present tense often covers both:

• a general present: doesn’t work well
• a current ongoing situation: isn’t working well

Because of σήμερα (today), English will often prefer:

• The air conditioner isn’t working well today.

But Greek δεν λειτουργεί itself does not force only one of those meanings. Context decides.

The given order is natural and standard, but Greek word order is more flexible than English.

You could also hear:

• Σήμερα το κλιματιστικό δεν λειτουργεί καλά.
• Το κλιματιστικό σήμερα δεν λειτουργεί καλά.
• Δεν λειτουργεί καλά σήμερα το κλιματιστικό.
  This last one is less neutral and depends more on context or emphasis.

The original sentence is a very normal, neutral way to say it.

A simple approximation for an English speaker is:

To klimatistikó den litourgí kalá símera

A few pronunciation notes:

• το sounds like to in top but shorter
• κλι in κλιματιστικό sounds like klee
• γεί in λειτουργεί sounds like GHEE / YEE depending on how detailed you get with Greek pronunciation, but for learners lee-toor-GHEE is a helpful approximation
• καλά = ka-LA
• σήμερα = SEE-me-ra

The stressed syllables are:

• κλιματιστικό
• λειτουργεί
• καλά
• σήμερα

In this sentence, κλιματιστικό is definitely a noun: air conditioner.

But related forms can also be used adjectivally in Greek in some contexts, because many Greek words can function in more than one way depending on structure.

For a learner, the safest thing here is simply:

• το κλιματιστικό = the air conditioner

That is the natural everyday meaning in this sentence.