Το κλιματιστικό στο σαλόνι είναι παλιό, αλλά ακόμα κάνει καλή δουλειά όταν έχει πολλή ζέστη.

Because both words are neuter singular.

• το κλιματιστικό = the air conditioner
• παλιό = old

In Greek, adjectives must agree with the noun they describe in gender, number, and case. Since κλιματιστικό is neuter singular nominative, the adjective also appears in the neuter singular nominative form:

• ο παλιός = masculine
• η παλιά = feminine
• το παλιό = neuter

So Το κλιματιστικό ... είναι παλιό means The air conditioner ... is old.

στο is a contraction of σε + το.

• σε = in / at / to
• το = the

So:

• σε το σαλόνι becomes στο σαλόνι

This is very common in Greek. A few similar contractions are:

• στον = σε τον
• στη = σε τη(ν)
• στην = σε την
• στους = σε τους

In this sentence, στο σαλόνι means in the living room.

In this sentence, it most naturally describes which air conditioner we are talking about:

• Το κλιματιστικό στο σαλόνι = The air conditioner in the living room

So στο σαλόνι modifies κλιματιστικό, not the whole clause.

That said, in real life the location also obviously helps imply where it works, but grammatically it is attached to the air conditioner.

σαλόνι is a neuter noun, so it takes το in the singular:

• το σαλόνι = the living room

It is one of many neuter nouns ending in -ι.

Because it is after σε, Greek uses the article in the contracted form:

• στο σαλόνι = in the living room

Greek uses the present tense here for a general present fact:

• είναι παλιό = it is old
• κάνει καλή δουλειά = it still does a good job

This is just like English when talking about something generally true or habitually true. The sentence is not describing one specific moment only; it is describing the air conditioner’s current condition and usual performance.

It is a very common expression meaning:

• does a good job
• works well
• performs well

Literally, it is makes/does good work, but in natural English you would usually translate it idiomatically.

So here:

• ακόμα κάνει καλή δουλειά = it still does a good job / it still works well

Because δουλειά is a feminine noun.

• η δουλειά = the work / the job

The adjective must agree with it, so Greek uses the feminine form:

• καλός = masculine
• καλή = feminine
• καλό = neuter

So:

• καλή δουλειά = good work / a good job

Here ακόμα means still.

So:

• αλλά ακόμα κάνει καλή δουλειά = but it still does a good job

Depending on context, ακόμα can also mean yet, even, or any more, but here still is clearly the right meaning.

You may also see ακόμη, which is a variant spelling/form. In many everyday contexts, ακόμα is very common in speech.

This is a normal Greek way to talk about hot weather.

• έχει ζέστη = it is hot
• literally: there is heat / it has heat

So:

• όταν έχει πολλή ζέστη = when it is very hot

Greek often uses έχει in weather-type expressions:

• έχει κρύο = it is cold
• έχει ζέστη = it is hot
• έχει αέρα = it is windy

This may feel unusual to an English speaker, but it is very natural Greek.

Because ζέστη is a feminine noun, and here πολλή is the feminine form of much/a lot of.

Compare:

• πολύ νερό = a lot of water / much water
  (νερό is neuter)
• πολλή ζέστη = a lot of heat / very hot weather
  (ζέστη is feminine)

This is a very useful distinction:

• πολύ can be an adverb meaning very
• πολλή is the feminine adjective meaning much / a lot of

In this sentence, πολλή goes with ζέστη, so it has to agree with it.

It is in the accusative, because έχω normally takes a direct object.

So in:

• έχει πολλή ζέστη

the phrase πολλή ζέστη functions grammatically like the object of έχει.

In practice, though, learners often just memorize έχει ζέστη as a fixed weather expression meaning it is hot.

Because Greek often does not need a subject pronoun, and in weather expressions it may not use one at all.

English says:

• when it is very hot

But Greek simply says:

• όταν έχει πολλή ζέστη

There is no separate word corresponding to English it here. The verb form and the expression itself are enough.

αλλά means but and introduces a contrast:

• είναι παλιό = it is old
• αλλά ακόμα κάνει καλή δουλειά = but it still does a good job

So the contrast is:

• it is old
• but despite that, it still works well

This is a very common use of αλλά.

Yes. Both would sound natural, but they are slightly different in tone.

• δουλεύει καλά = it works well
• κάνει καλή δουλειά = it does a good job

The version in your sentence sounds a little more idiomatic and expressive. It emphasizes performance, not just functioning.

So:

• Το κλιματιστικό ... δουλεύει καλά = perfectly natural
• Το κλιματιστικό ... κάνει καλή δουλειά = also natural, with a slightly more evaluative feel

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

This sentence starts with the topic:

• Το κλιματιστικό στο σαλόνι = The air conditioner in the living room

Then it gives two pieces of information about it:

• είναι παλιό = is old
• αλλά ακόμα κάνει καλή δουλειά = but still does a good job

And finally the condition:

• όταν έχει πολλή ζέστη = when it is very hot

So the structure is very natural and easy to follow. Greek could rearrange parts for emphasis, but this version is straightforward and neutral.

They are:

• κλιματιστικό = air conditioner
• σαλόνι = living room
• είμαι = to be
• παλιός, παλιά, παλιό = old
• αλλά = but
• ακόμα = still
• κάνω = to do / make
• καλός, καλή, καλό = good
• δουλειά = work / job
• όταν = when
• έχω = to have
• πολύς, πολλή, πολύ = much / many
• ζέστη = heat / hot weather

For learners, it is especially useful to remember adjectives in all three gender forms, like:

• παλιός, παλιά, παλιό
• καλός, καλή, καλό
• πολύς, πολλή, πολύ

because Greek adjectives change form depending on the noun they describe.