Η συναυλία ήταν τρομερή.

A natural pronunciation is:

i sinafvlía ítan tromeRÍ
More carefully: [i sinafˈlia ˈitan tromeˈri]

A few useful points:

• Η is pronounced i
• συναυλία is stressed on -λί-
• ήταν is stressed on the first syllable: Í-tan
• τρομερή is stressed on the last syllable: tro-me-RÍ

Depending on the sound that follows, αυ can sound like av or af. In συναυλία, it is pronounced av because the next sound is voiced.

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

So:

• Η συναυλία = the concert

Greek articles change for gender, number, and case, unlike English the, which always stays the same.

In Greek, every noun has grammatical gender: masculine, feminine, or neuter.

συναυλία is a feminine noun, so it takes:

• the feminine article: η
• a feminine adjective: τρομερή

This is grammatical gender, not biological gender. A thing like concert can still be feminine in Greek.

Because adjectives in Greek must agree with the noun they describe.

Since συναυλία is:

• feminine
• singular
• nominative

the adjective must also be:

• feminine
• singular
• nominative

So you get:

• masculine: τρομερός
• feminine: τρομερή
• neuter: τρομερό

Here, τρομερή matches συναυλία.

ήταν is the past tense form of είμαι (to be).

So:

• είναι = is / are
• ήταν = was / were

In this sentence, ήταν means was:

• Η συναυλία ήταν τρομερή. = The concert was amazing / terrible / terrific, depending on context.

In the past tense of είμαι, Greek commonly uses the same form ήταν for different persons in the singular and plural in everyday language.

For example:

• ήταν = I was / you were / he was / she was / it was / they were
  depending on context

So Greek relies more on the subject or context to make the meaning clear.

It can mean either, depending on context and tone.

Originally, τρομερός / τρομερή / τρομερό can mean something like:

• terrible
• frightening
• tremendous

But in everyday speech it very often means:

• great
• amazing
• fantastic

So Η συναυλία ήταν τρομερή often means:

• The concert was amazing

But in the right context, it could also mean:

• The concert was terrible

Context tells you which meaning is intended.

Yes. If you want to avoid the possible terrible/amazing ambiguity of τρομερή, you could say:

• Η συναυλία ήταν καταπληκτική. = The concert was amazing.
• Η συναυλία ήταν φανταστική. = The concert was fantastic.
• Η συναυλία ήταν υπέροχη. = The concert was wonderful.

These are often clearer for learners.

Yes. Greek word order is more flexible than English.

The neutral order is:

• Η συναυλία ήταν τρομερή.

But you may also hear:

• Τρομερή ήταν η συναυλία.
• Η συναυλία τρομερή ήταν.
  though this can sound more marked or stylistic

Changing the order usually changes the emphasis, not the basic meaning.

In normal Greek, you usually need the article with a definite noun like this.

So:

• Η συναυλία = the concert
• Μια συναυλία = a concert

Leaving the article out would usually sound incomplete or unnatural in an ordinary sentence.

You would say:

• Μια συναυλία ήταν τρομερή.

Here:

• μια = a / one
• συναυλία stays the same
• τρομερή still agrees with the feminine noun

Greek uses written accents to show the stressed syllable.

In συναυλία, the stress falls on -λί-:

• συ-ναυ-λί-α

In τρομερή, the stress falls on the final syllable:

• τρο-με-ρή

Stress matters in Greek pronunciation, so the accent marks are important and should be learned as part of the word.

The letter combination αυ is not always pronounced the same way.

It is usually pronounced:

• av before a voiced sound or a vowel
• af before a voiceless sound

In συναυλία, the next sound is λ, which is voiced, so αυ is pronounced av.

So συναυλία sounds roughly like:

• sinavlía

In Modern Greek, η is pronounced i.

So the article Η sounds exactly like:

• η
• ι
• υ
• ει
• οι

in terms of pronunciation

These spellings may sound the same in Modern Greek, even though they are written differently.

You can, but it may sound a bit unnatural depending on context.

τρομερή already has a strong meaning, so Greek speakers are often more likely to say:

• Η συναυλία ήταν πάρα πολύ καλή. = The concert was very good.
• Η συναυλία ήταν καταπληκτική. = The concert was amazing.
• Η συναυλία ήταν πραγματικά τρομερή. = The concert was really amazing/terrible.

So πολύ τρομερή is possible, but not always the most natural choice.

You put δεν before the verb:

• Η συναυλία δεν ήταν τρομερή.

That means:

• The concert wasn’t amazing
• or The concert wasn’t terrible

again depending on how τρομερή is being understood in context.

Yes. It is short, correct, and completely natural.

It follows a very common Greek pattern:

• article + noun + ήταν + adjective

For example:

• Η ταινία ήταν ωραία. = The film was nice.
• Το φαγητό ήταν καλό. = The food was good.
• Η συναυλία ήταν τρομερή. = The concert was amazing / terrible.

So it is a very useful model sentence for beginners.