Η συναυλία ήταν αξέχαστη.

A natural pronunciation is roughly:

ee see-nav-LEE-a EE-tan a-KSE-ha-stee

A more accurate breakdown:

• Η = ee
• συναυλία = see-nav-LEE-a
• ήταν = EE-tan
• αξέχαστη = a-KSE-ha-stee

A few helpful sound notes:

• η sounds like ee
• υ here sounds like i/ee
• ξ sounds like ks
• χ is not like English ch in church. It is a throaty sound, like the ch in German Bach or Scottish loch

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

It matches συναυλία, which is a feminine singular noun.

So:

• η συναυλία = the concert

A very common beginner point: even though Η looks like a capital English H, in Greek it is a completely different letter and is pronounced ee, not h.

Yes. Συναυλία is a feminine noun in Greek.

In Greek, nouns have grammatical gender:

• masculine
• feminine
• neuter

Because συναυλία is feminine singular, the article and adjective must also be feminine singular:

• η συναυλία
• αξέχαστη

This is called agreement.

Because adjectives in Greek change form to agree with the noun they describe.

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

• masculine: αξέχαστος
• feminine: αξέχαστη
• neuter: αξέχαστο

So:

• ο αγώνας ήταν αξέχαστος = the match was unforgettable
• η συναυλία ήταν αξέχαστη = the concert was unforgettable
• το ταξίδι ήταν αξέχαστο = the trip was unforgettable

Ήταν is the past form of είμαι (to be).

In this sentence, it means was.

So:

• είναι = is
• ήταν = was

More specifically, ήταν is the imperfect form, but in simple sentences like this, you can usually think of it as was.

Examples:

• Η συναυλία είναι αξέχαστη. = The concert is unforgettable.
• Η συναυλία ήταν αξέχαστη. = The concert was unforgettable.

In Greek, the same form ήταν can be used for:

• I was
• you were
• he/she/it was
• they were

The subject usually makes it clear.

Examples:

• Η συναυλία ήταν αξέχαστη. = The concert was unforgettable.
• Οι συναυλίες ήταν αξέχαστες. = The concerts were unforgettable.

So Greek often uses the same past form for several persons and numbers.

The accent marks show you which syllable is stressed.

In this sentence:

• συναυλία → stress on -λί-
• ήταν → stress on ή-
• αξέχαστη → stress on -ξέ-

So you say:

• συ-ναυ-ΛΙ-α
• Η-ταν
• α-ΞΕ-χα-στη

These accent marks are very important in Greek because they show pronunciation clearly.

This is the most neutral and natural word order.

Greek word order is more flexible than English, so you may sometimes see variations for emphasis, such as:

• Αξέχαστη ήταν η συναυλία.

That puts more emphasis on unforgettable.

But for a learner, Η συναυλία ήταν αξέχαστη is the safest standard pattern:

• article + noun + was + adjective

Usually, no—not in a normal sentence like this.

Greek often uses the definite article where English might or might not use the. Here, η συναυλία sounds natural as the concert.

Leaving out the article would usually sound incomplete or unnatural in this context.

So the normal sentence is:

• Η συναυλία ήταν αξέχαστη.

No. Αξέχαστη is only the feminine singular form.

The adjective itself is the same basic word, but it changes ending depending on the noun:

• αξέχαστος = masculine singular
• αξέχαστη = feminine singular
• αξέχαστο = neuter singular

Plural forms also change:

• αξέχαστοι
• αξέχαστες
• αξέχαστα

So you choose the form that matches the noun.

If you want to say The concerts were unforgettable, you would say:

Οι συναυλίες ήταν αξέχαστες.

Changes:

• Η → Οι for plural the
• συναυλία → συναυλίες
• ήταν stays the same
• αξέχαστη → αξέχαστες to agree with the feminine plural noun

This is a great example of how Greek articles, nouns, and adjectives all work together.