Το διαμέρισμά μας είναι πιο φωτεινό από το παλιό.

In Greek, possessed nouns normally still take the definite article. So:

• το διαμέρισμά μας = our apartment
• literally, it looks like the apartment our, but that is just normal Greek structure

This is very common:

• το σπίτι μου = my house
• η αδελφή της = her sister
• οι φίλοι μας = our friends

So the το is not extra in Greek; it is the standard way to say it.

Greek weak possessive forms such as μου, σου, του, της, μας, σας, τους usually come after the noun:

• το διαμέρισμά μας = our apartment
• το βιβλίο μου = my book

So μας here means our, but it is placed after the noun rather than before it, unlike English.

This happens because μας is an unstressed clitic word. When a noun like διαμέρισμα is followed by one of these clitics, Greek often adds a written accent to help preserve the stress pattern:

• διαμέρισμα
• διαμέρισμά μας

This is a standard spelling rule. You will see the same thing in examples like:

• το πρόσωπό μου
• το όνομά σου

So the extra accent is not changing the meaning; it reflects how the phrase is pronounced and stressed.

Because it has to agree with διαμέρισμα.

The noun διαμέρισμα is:

• neuter
• singular
• nominative here, because it is the subject of the sentence

So the adjective must match it:

• φωτεινός = masculine
• φωτεινή = feminine
• φωτεινό = neuter

Since διαμέρισμα is neuter singular, the correct form is φωτεινό.

Yes. πιο + adjective is a very common way to form the comparative in Greek.

So:

• φωτεινό = bright
• πιο φωτεινό = brighter / more bright

This is one of the main patterns for comparisons:

• πιο μεγάλο = bigger
• πιο όμορφη = more beautiful
• πιο ακριβό = more expensive

So πιο φωτεινό means brighter.

Yes. Both are possible.

You can form the comparative in two common ways:

• πιο φωτεινό
• φωτεινότερο

Both mean brighter.

In everyday Greek, πιο + adjective is extremely common and often feels simpler. So your sentence could also be:

• Το διαμέρισμά μας είναι φωτεινότερο από το παλιό.

That would still be correct.

Here από means than in a comparison.

So:

• πιο φωτεινό από... = brighter than...

In other contexts, από can also mean from, but in this sentence its job is to introduce the second part of the comparison.

Examples:

• είναι μεγαλύτερο από το άλλο = it is bigger than the other one
• είμαι πιο κουρασμένος από χτες = I am more tired than yesterday

Because παλιό here stands for the old one / the old apartment, so it needs the article το.

Greek often uses the article with an adjective when the noun is understood from context:

• το παλιό = the old one
• understood here as the old apartment

So από το παλιό means than the old one.

If you wanted to say the full noun, you could say:

• από το παλιό διαμέρισμα

But Greek often leaves the noun out when it is obvious.

It can mean both, depending on context.

Grammatically, το παλιό is just the old one. But because the sentence is already talking about an apartment, Greek speakers understand that the missing noun is διαμέρισμα.

So in context:

• το παλιό = the old one
• more specifically, the old apartment

This kind of omission is very normal in Greek.

Είναι is the 3rd person singular of είμαι = to be.

The subject is το διαμέρισμά μας = our apartment, which is singular, so Greek uses singular είναι:

• Το διαμέρισμά μας είναι... = Our apartment is...

If the subject were plural, you would still usually use είναι, because in Modern Greek the form είναι is used for both singular he/she/it is and plural they are:

• Το διαμέρισμά μας είναι φωτεινό.
• Τα διαμερίσματά μας είναι φωτεινά.

This order is very natural:

• Το διαμέρισμά μας είναι πιο φωτεινό από το παλιό.

Greek word order is somewhat flexible, but not completely free. You can sometimes move things around for emphasis, though the given sentence is the most neutral and natural way to say it.

For example, you might also hear:

• Το διαμέρισμά μας είναι φωτεινότερο από το παλιό.

But for a learner, the original order is the safest and most standard pattern to use.