Η αντίκα είναι αυθεντική.

Η is the feminine singular definite article, so it means the.

In Η αντίκα είναι αυθεντική, the noun αντίκα is treated as a feminine singular noun, so it takes η. Greek articles change for gender, number, and case, so the article gives you useful grammatical information that English the does not.

So:

η αντίκα = the antique

Here, αντίκα is a noun. It means an antique item or antique object.

That can be confusing for English speakers, because in English antique can be either:

• a noun: This antique is expensive
• an adjective: an antique chair

In this Greek sentence, αντίκα is the subject of the sentence, so it is clearly being used as a noun: The antique is authentic.

Because that is simply its grammatical gender in Greek.

Greek nouns have grammatical gender, and that gender does not always match anything logical or natural from an English point of view. Since αντίκα is feminine, words that agree with it must also be feminine.

That is why you get:

• η αντίκα
• αυθεντική

not masculine or neuter forms.

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

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

• masculine: αυθεντικός
• feminine: αυθεντική
• neuter: αυθεντικό

So in this sentence:

• η αντίκα = feminine singular
• αυθεντική = feminine singular

That agreement is required in Greek.

Είναι means is here. It is the present-tense form of the verb είμαι, meaning to be.

In this sentence, it links the subject and the adjective:

• Η αντίκα = the antique
• είναι = is
• αυθεντική = authentic / genuine

So the structure is very similar to English:

The antique is authentic.

A useful thing to know is that είναι is also used for they are in Modern Greek, so context tells you whether it is singular or plural.

In normal Modern Greek, no. You normally keep είναι in a sentence like this.

So:

• Η αντίκα είναι αυθεντική = normal and correct

Leaving out είναι would sound incomplete in ordinary speech, except perhaps in special contexts such as titles, labels, notes, or very compressed style.

Usually it is very close, but in many contexts it can also feel like genuine, real, or original, not fake.

So for an object like an antique, αυθεντική strongly suggests that it is not an imitation or not a fake.

That is a very natural word choice for antiques, art, documents, and similar things.

Greek word order is more flexible than English word order, although the version you were given is the most neutral and straightforward:

Η αντίκα είναι αυθεντική.

That is the ordinary, unmarked way to say it.

You can sometimes change the order for emphasis. For example:

Αυθεντική είναι η αντίκα.

This sounds more emphatic, something like It is the antique that is authentic or The antique is the authentic one depending on context.

So yes, the order can change, but the original sentence is the standard one for a learner to use first.

A rough pronunciation for an English speaker is:

ee an-TEE-ka EE-ne af-then-dee-KEE

A few helpful details:

• Η sounds like ee
• αντίκα is stressed on -τί-
• είναι is stressed on εί-
• αυθεντική is stressed on the last syllable, -κή

Also, in αυθεντική, the αυ is pronounced af because it comes before the voiceless sound θ.

The accent marks show which syllable is stressed.

In your sentence:

• αντίκα → stress on τί
• είναι → stress on εί
• αυθεντική → stress on κή

This matters because stress is an important part of Greek pronunciation, and moving the stress can make a word sound wrong or even change meaning in some cases.

The article η has no written accent because it is only one syllable.