Η εγχείρηση κράτησε δύο ώρες.

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

It agrees with εγχείρηση in:

• gender: feminine
• number: singular
• case: nominative

So Η εγχείρηση = the operation / the surgery.

Because Greek nouns have grammatical gender, and εγχείρηση happens to be a feminine noun.

That does not mean the operation itself is somehow biologically feminine. It is just a grammatical category. So you say:

• η εγχείρηση
• not ο εγχείρηση or το εγχείρηση

The stress is on χεί: εγ-ΧΕΙ-ρη-ση.

A rough pronunciation for an English speaker is: eng-HEE-ree-see

A couple of useful points:

• γχ is not pronounced like English g in go
• it sounds more like ngh / ngkh
• χ before ει is a softer sound than English k

You do not need to pronounce it perfectly right away; getting the stress in the right place is the most important first step.

Yes, the basic verb κρατάω / κρατώ often means hold, keep, or retain. But it also has a very common extended meaning: to last or to take (a certain amount of time).

So in this sentence:

• Η εγχείρηση κράτησε δύο ώρες means
• The operation lasted two hours

This is very natural Greek. It works a bit like English sentences such as The meeting took two hours.

Κράτησε is the aorist form, 3rd person singular.

That means it describes the event as a completed whole in the past:

• the operation lasted two hours

Its dictionary form is:

• κρατάω or κρατώ

So the relationship is roughly:

• κρατάω / κρατώ = I hold / I last
• κράτησε = it held / it lasted

The aorist is used because the speaker is presenting the operation as a finished event with a complete duration.

So:

• κράτησε δύο ώρες = it lasted two hours as a complete past fact

If you used κρατούσε, that would be the imperfect, which gives more of an ongoing or background sense:

• Η εγχείρηση κρατούσε ακόμη... = The operation was still going on...

So in this sentence, κράτησε is the normal choice.

Because Greek often expresses duration with the accusative directly, without a preposition.

So:

• κράτησε δύο ώρες literally has the structure
• lasted two hours

This is very similar to English, where you can also say:

• It lasted two hours without needing for.

It is functioning as an accusative of duration.

More specifically:

• δύο = two
• ώρες = hours

Ώρες is plural because the sentence says two hours.
Also, with this noun, the nominative plural and accusative plural happen to look the same: ώρες.

Usually, no. Δύο is generally treated as an indeclinable numeral, so it stays the same here.

So you get:

• δύο ώρες
• δύο μέρες
• δύο βιβλία

The noun after it changes as needed, but δύο itself normally does not.

Yes. Greek word order is fairly flexible.

The sentence as given:

• Η εγχείρηση κράτησε δύο ώρες is a neutral, natural way to say it.

But you could also say:

• Δύο ώρες κράτησε η εγχείρηση — this emphasizes two hours
• Κράτησε δύο ώρες η εγχείρηση — also possible, especially in speech

So the meaning stays basically the same, but the emphasis can shift.

In a normal full sentence, η is the natural choice.

Greek uses the definite article more often than English, especially with a specific noun like this:

• Η εγχείρηση κράτησε δύο ώρες

Leaving the article out would sound unnatural in ordinary speech, unless you were writing a headline, note, or some very compressed style.

Yes, absolutely.

Διήρκεσε also means lasted, and the sentence is completely correct:

• Η εγχείρηση διήρκεσε δύο ώρες

The difference is mostly one of style:

• κράτησε = very common, everyday Greek
• διήρκεσε = a bit more formal or bookish

So both are right, but κράτησε is especially common in ordinary conversation.