Η αντίκα είναι από ορείχαλκο, μόνο που είναι φθαρμένη.

Because αντίκα is treated as a feminine noun in Greek. Greek nouns have grammatical gender, and you usually have to learn the gender along with the word.

So:

• η αντίκα = the antique

Even if the object itself is not biologically female, the noun is still grammatically feminine.

Here από means made of or from in the sense of material.

So:

• είναι από ορείχαλκο = it is made of brass

Greek often uses από in this way to show what something is made from:

• από ξύλο = made of wood
• από γυαλί = made of glass
• από χρυσό / από χρυσάφι = made of gold

After από, Greek normally uses the accusative case.

The noun ορείχαλκος is a masculine noun, and its accusative singular is ορείχαλκο.

In this sentence there is no article, which is very natural when talking about material in a general sense:

• από ορείχαλκο = made of brass

You could also see:

• από τον ορείχαλκο

but that is less natural here unless you are referring to some specific brass already mentioned.

Because the sentence has two separate statements joined together:

• Η αντίκα είναι από ορείχαλκο
• μόνο που είναι φθαρμένη

Greek often repeats the verb where English might also repeat it, though sometimes English learners expect something more compact. The second είναι is necessary here because φθαρμένη is the predicate adjective of a new clause.

So the structure is basically:

• The antique is made of brass,
• except that / only it is worn.

μόνο που is an expression that often means something like:

• except that
• only
• the only thing is that
• but

In this sentence, it introduces a small limitation or drawback:

• Η αντίκα είναι από ορείχαλκο, μόνο που είναι φθαρμένη. = The antique is made of brass, only / except that it is worn.

It is not a word-for-word only that in the English sense. It is an idiomatic connector.

Because it agrees with η αντίκα, which is feminine singular.

Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Since αντίκα is feminine singular, the adjective must also be feminine singular:

• masculine: φθαρμένος
• feminine: φθαρμένη
• neuter: φθαρμένο

So:

• η αντίκα είναι φθαρμένη

φθαρμένη means something like:

• worn
• worn out
• damaged by use
• weathered
• showing signs of age or friction

For an antique, it usually suggests physical wear rather than complete destruction. It can imply that the surface, finish, or condition is no longer perfect.

In this sentence, αντίκα is a noun meaning antique.

However, in modern Greek it can also sometimes behave like an indeclinable adjective in certain contexts, especially in more informal or commercial language, such as:

• έπιπλο αντίκα = antique piece of furniture

But in your sentence:

• Η αντίκα clearly means the antique as a noun.

A rough pronunciation guide is:

ee an-DEE-ka EE-ne a-PO o-REE-hal-ko, MO-no poo EE-ne fthar-ME-ni

A few notes:

• η sounds like ee
• είναι is pronounced EE-ne
• θ is like the th in think
• χ in ορείχαλκο is like the ch in German Bach or Scottish loch
• φθ at the start of φθαρμένη can be tricky; it is roughly fth

Because μόνο που introduces a new clause that adds a qualification or contrast. The comma helps separate:

• the main statement: Η αντίκα είναι από ορείχαλκο
• the added limitation: μόνο που είναι φθαρμένη

This is very natural punctuation in Greek.

Normally, no. That would sound like you are identifying the antique as brass itself, which is not the usual way to talk about material.

Greek generally prefers:

• είναι από ορείχαλκο = it is made of brass

That is the standard pattern for materials.

It is a correct and standard word, but it is a bit more specific and formal than some very basic vocabulary. It means brass.

A learner might not meet it as early as words like:

• ξύλο = wood
• σίδερο = iron
• γυαλί = glass

But it is a normal word, especially in descriptions of objects, decoration, antiques, and materials.

Not always, but it often introduces a qualification, restriction, or small problem.

For example, it can mean:

• everything is fine, except that...
• yes, but...

So in this sentence, it adds a negative detail:

• the antique is made of brass, but it is worn

The expression itself is not inherently negative, but it very often signals a contrast or drawback.