Ο ορείχαλκος είναι ακριβός.

This is very normal in Greek. Greek often uses the definite article with nouns when talking about something in a general sense, including materials and substances.

So:

Ο ορείχαλκος είναι ακριβός.

looks literally like The brass is expensive, but the natural meaning is simply Brass is expensive.

This is one of the places where Greek uses the article more often than English.

Ορείχαλκος is masculine singular.

You can tell mainly from the article:

• ο = masculine singular nominative
• η = feminine singular nominative
• το = neuter singular nominative

So ο ορείχαλκος must be masculine singular.

The ending -ος often belongs to masculine nouns, although not always, so the article is the safest clue.

Because adjectives in Greek must agree with the noun they describe in gender, number, and case.

Here, ο ορείχαλκος is:

• masculine
• singular
• nominative

So the adjective also appears as masculine singular nominative:

• ακριβός = masculine
• ακριβή = feminine
• ακριβό = neuter

For example:

• Ο ορείχαλκος είναι ακριβός.
• Η βενζίνη είναι ακριβή.
• Το μέταλλο είναι ακριβό.

It is in the nominative case, because it is the subject of the sentence.

In Ο ορείχαλκος είναι ακριβός, the subject is ο ορείχαλκος, and the predicate adjective ακριβός is also in the nominative, because with είναι Greek normally uses the nominative for this kind of sentence.

In normal standard Greek, yes, είναι is needed.

So the basic full sentence is:

Ο ορείχαλκος είναι ακριβός.

You may sometimes see the verb omitted in headlines, notes, or very informal styles, but that is not the normal pattern learners should copy. In regular speech and writing, keep είναι.

Yes. Greek word order is more flexible than English word order.

The neutral order here is:

Ο ορείχαλκος είναι ακριβός.

But Greek can also change the order for emphasis, for example:

Ακριβός είναι ο ορείχαλκος.

That would sound more like It’s brass that is expensive or Brass is expensive with emphasis on expensive.

For learners, the original order is the safest and most neutral one.

A rough guide is:

o o-RI-hal-kos EE-ne a-kri-VOS

Stress falls on:

• ορείχαλκος → -ρεί-
• είναι → εί-
• ακριβός → -βός

A few pronunciation notes:

• ει is pronounced like ee
• χ is not like English k; it is a rough throat sound, similar to the ch in German Bach
• β in Modern Greek sounds like English v

So ακριβός sounds like a-kri-VOS, not a-kri-BOS.

Modern Greek normally writes an accent mark on polysyllabic words to show where the stress falls.

So you get:

• ορείχαλκος
• είναι
• ακριβός

But ο is only one syllable, so it normally does not take a written accent.

That is why the article appears as ο, not ό.

Not always, but in this sentence that is clearly the meaning.

Ακριβός often means:

• expensive
• sometimes dear or beloved, depending on context

For example:

• Το αυτοκίνητο είναι ακριβό. = The car is expensive.
• Ο ακριβός μου φίλος can mean my dear friend

But with a material like ορείχαλκος, the meaning is definitely expensive.

Because ορείχαλκος here refers to the material brass, which is being treated as a mass noun, not a countable item.

Just like English normally says brass rather than a brass, Greek does not use ένας here.

If you wanted to talk about a piece or object made of brass, you would normally use another expression, such as:

• ένα κομμάτι ορείχαλκου = a piece of brass
• ένα ορειχάλκινο αντικείμενο = a brass object

So in this sentence, the generic form with the article is the natural one:

Ο ορείχαλκος είναι ακριβός.