Η πόλη χτίστηκε πριν από έναν αιώνα.

Because this is a passive sentence. In a passive sentence, the thing receiving the action becomes the subject of the sentence, and subjects take the nominative case.

So:

• Η πόλη χτίστηκε = The city was built
• Έχτισαν την πόλη = They built the city

In the second sentence, την πόλη is the object, so it is accusative.

Χτίστηκε is the aorist passive, third person singular, of χτίζω or κτίζω (to build).

So it means:

• was built

The aorist usually refers to a completed event in the past.

Because Greek often expresses the English idea was + past participle with one verb form.

English:

• was built

Greek:

• χτίστηκε

So the meaning of was built is packed into a single word.

It is a very common aorist passive pattern. In this sentence, χτίστηκε clearly shows a past passive meaning: was built.

That said, it is best to learn full verb forms rather than assuming every verb will behave exactly the same way. Still, recognizing -στηκε as a common passive past ending is very useful.

Here πριν από means ago with a time expression.

So:

• πριν από έναν αιώνα = a century ago

Literally, it is closer to before one century, but in natural English we say a century ago.

Because αιώνας is a masculine noun.

The forms are:

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

In the accusative singular, masculine ένας becomes έναν. So:

• έναν αιώνα = one century

Because it is in the accusative singular:

• dictionary form: αιώνας
• accusative singular: αιώνα

This happens because από takes the accusative, and the phrase πριν από is followed by an accusative time expression.

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

The sentence:

• Η πόλη χτίστηκε πριν από έναν αιώνα

is a neutral way to say it.

You could also say:

• Πριν από έναν αιώνα χτίστηκε η πόλη

This puts more emphasis on the time phrase πριν από έναν αιώνα. The basic meaning stays the same.

Yes. Both spellings are accepted:

• χτίζω / χτίστηκε
• κτίζω / κτίστηκε

In everyday Modern Greek, χτίζω is very common. You should recognize both forms when reading.