Το υπνοδωμάτιο αερίζεται κάθε πρωί, αλλά το σαλόνι μόνο όταν έχει ζέστη.

Αερίζεται is the middle/passive form of the verb αερίζω.

In this sentence, it means is aired / gets aired / is ventilated. Greek often uses this kind of form when the focus is on what happens to something, not on who does it.

So:

• αερίζω = I air, I ventilate
• αερίζεται = it is aired / it gets aired

That is why Το υπνοδωμάτιο αερίζεται κάθε πρωί means The bedroom is aired every morning.

In everyday Greek, this can sound very natural even when no person is mentioned.

Both nouns are neuter singular, so they take the article το.

• το υπνοδωμάτιο = the bedroom
• το σαλόνι = the living room / lounge

Their forms here are in the nominative singular, because they are the subjects of the sentence.

A learner may want to remember them as full vocabulary items with the article:

• το υπνοδωμάτιο
• το σαλόνι

That helps you remember both the meaning and the gender.

In Greek, the definite article is used very often, including in general statements where English may sound a little more flexible.

Here, Το υπνοδωμάτιο αερίζεται κάθε πρωί does not necessarily mean one specific bedroom in an overly narrow sense. It can simply refer to the bedroom as the room being discussed in the house.

Likewise:

• το σαλόνι = the living room

Greek usually keeps the article with nouns like this, so omitting it would sound unnatural in this sentence.

Κάθε πρωί means every morning.

This is a very common Greek time expression:

• κάθε μέρα = every day
• κάθε βράδυ = every evening/night
• κάθε πρωί = every morning

Here, πρωί is being used as a time expression, not as an adjective. So κάθε πρωί is the normal phrase.

Also, κάθε does not change form here. It is the regular word used for every / each in expressions like this.

Μόνο όταν means only when.

The two words work together like this:

• μόνο = only
• όταν = when

So:

• το σαλόνι μόνο όταν έχει ζέστη
  = the living room only when it’s hot

This means the living room is not aired every morning; it is aired only in the condition that the weather is hot.

This is a very important Greek pattern.

Greek often uses:

• έχει ζέστη = it is hot / there is heat
• έχει κρύο = it is cold
• έχει ήλιο = it is sunny

Literally, έχει ζέστη is something like it has heat, but in natural English we translate it as it is hot.

So in this sentence:

• όταν έχει ζέστη = when it’s hot

This is a fixed and very common weather expression, so it is best learned as a whole chunk.

Yes. In the second clause, the verb αερίζεται is understood and left out because it would be repetitive.

Full version:

• Το υπνοδωμάτιο αερίζεται κάθε πρωί, αλλά το σαλόνι αερίζεται μόνο όταν έχει ζέστη.

But Greek, like English, often omits repeated material when it is obvious:

• ..., αλλά το σαλόνι μόνο όταν έχει ζέστη.

English does the same:

• The bedroom is aired every morning, but the living room only when it’s hot.

So this ellipsis is very natural.

Αλλά means but.

It introduces a contrast between the two parts:

• the bedroom is aired every morning
• the living room is aired only when it’s hot

So the contrast is between:

• regularly / every morning and
• only under a specific condition

It is a straightforward, common coordinating conjunction.

In this kind of expression, πρωί functions as a time expression, and Greek often uses the accusative for duration or time-related adverbial meanings.

So κάθε πρωί is an adverbial expression meaning every morning.

A learner does not always need to analyze it deeply at first, but it helps to know that Greek commonly uses the accusative in time expressions like this.

Normally, it means when the weather is hot or when it is hot in general, not specifically that the living room itself is hot.

The expression έχει ζέστη is commonly used about the general temperature or weather conditions.

So the idea is:

• the living room is aired only during hot weather

If Greek wanted to be more explicit about the room itself being hot, it would usually be phrased differently.

Το υπνοδωμάτιο is a standard and very common word for bedroom.

You may also hear η κρεβατοκάμαρα, which also means bedroom. In many contexts, both are possible, though υπνοδωμάτιο is very neutral and standard.

In this sentence, το υπνοδωμάτιο is perfectly natural.

Αερίζεται is pronounced roughly like a-e-REE-ze-te.

The ending -εται is a common third person singular middle/passive present ending in Modern Greek.

So:

• αερίζεται = it is aired / it gets aired

The ζ sound belongs to the verb stem αερίζ-, and -εται is the grammatical ending.

If you compare:

• αερίζω = I air
• αερίζεις = you air
• αερίζεται = it is aired / it gets aired

you can start to recognize the pattern more easily.