Ήταν μια εκπληκτική εμπειρία.

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

More specifically, it is the imperfect form, used for was / were in many contexts. In this sentence, it corresponds to was.

A useful thing to know is that ήταν can be:

• 3rd person singular: he/she/it was
• 3rd person plural: they were

Greek relies on context to tell you which one is meant.

Greek often leaves out subject pronouns when they are not needed. This is very normal.

So instead of saying something like It was an amazing experience, Greek can simply say:

• Ήταν μια εκπληκτική εμπειρία.

If you wanted to make the subject explicit, you could say:

• Αυτό ήταν μια εκπληκτική εμπειρία. = That/It was an amazing experience.

But in everyday Greek, leaving it out is completely natural.

Μια is the feminine singular indefinite article, meaning a / an.

Greek articles must match the noun’s gender, number, and case. Since εμπειρία (experience) is:

• feminine
• singular

the correct article is μια.

The basic indefinite articles are:

• ένας = masculine
• μια / μία = feminine
• ένα = neuter

Greek does not change the indefinite article based on the following sound the way English does with a and an.

So:

• English: a book, an experience
• Greek: still just μια for a feminine noun

The choice depends on the noun’s gender, not on whether the next sound is a vowel.

Because the adjective must agree with the noun it describes.

Here, εκπληκτική describes εμπειρία, so it must also be:

• feminine
• singular
• nominative

The dictionary form of this adjective is usually shown as:

• εκπληκτικός, εκπληκτική, εκπληκτικό
  (amazing)

So in this sentence:

• εκπληκτική εμπειρία = feminine singular form

It is in the nominative case.

After είμαι (to be), Greek normally uses the nominative for the noun phrase that identifies or describes something.

So all three words here are nominative feminine singular:

• μια
• εκπληκτική
• εμπειρία

This is similar to how in grammar terms English says a sentence like It was an amazing experience, where an amazing experience is the complement of was.

Yes. Ήτανε is a very common variant in spoken Greek.

So both are possible:

• Ήταν μια εκπληκτική εμπειρία.
• Ήτανε μια εκπληκτική εμπειρία.

In general:

• ήταν is the more neutral and standard written form
• ήτανε is common in speech and informal style

They mean the same thing.

Yes, Greek adjectives can often come before or after the noun.

So you may see:

• μια εκπληκτική εμπειρία
• μια εμπειρία εκπληκτική

But they do not sound equally neutral.

The most natural, ordinary version here is:

• μια εκπληκτική εμπειρία

Putting the adjective after the noun can sound:

• more emphatic
• more literary
• less neutral in everyday speech

So for a learner, the safest choice here is the original order.

Greek word order is more flexible than English, but not all orders sound equally natural.

The neutral version is:

• Ήταν μια εκπληκτική εμπειρία.

You could change the order for emphasis, for example:

• Μια εκπληκτική εμπειρία ήταν.

But that sounds marked, as if you are stressing what it was.

So even though Greek allows flexibility, the original sentence is the most normal everyday phrasing.

The accent marks show which syllable is stressed.

For example:

• Ήταν
• εκπληκτική
• εμπειρία

This is important in Greek because stress is part of the correct pronunciation and sometimes helps distinguish forms.

You will notice that μια is usually written without an accent in everyday modern Greek spelling, even though you may also encounter μία.

Yes, they are basically the same word.

In modern Greek:

• μια is the most common spelling in everyday use
• μία also exists and may appear in more careful, formal, or emphatic writing

In this sentence, μια is perfectly normal and natural.

So:

• Ήταν μια εκπληκτική εμπειρία.
• Ήταν μία εκπληκτική εμπειρία.

Both are possible, but μια is the more usual choice.