Έλεγξαν όχι μόνο διάτρηση αλλά και χαρακτηριστικό ασφαλείας.

Έλεγξαν is the aorist active, 3rd person plural form of ελέγχω (to check, to inspect, to verify).

So it means they checked.

A few useful points:

• 3rd person plural = they
• aorist = a completed action, viewed as a whole
• In this sentence, it presents the checking as a finished event: they checked ...

Greek often drops subject pronouns when the verb ending already shows who the subject is.

So:

• Έλεγξαν already means they checked
• adding αυτοί (they) would usually only be done for emphasis or contrast

This is very normal in Greek. English usually needs they, but Greek often does not.

This is the Greek equivalent of not only ... but also ...

In your sentence:

• όχι μόνο διάτρηση
• αλλά και χαρακτηριστικό ασφαλείας

So the structure is:

• όχι μόνο X αλλά και Y
• not only X but also Y

It is a very common pattern in Greek, just as in English.

They are both functioning as the direct objects of Έλεγξαν (they checked), so they are in the accusative case.

However, there is an important detail:

• διάτρηση is a feminine noun whose nominative and accusative singular are the same in form
• χαρακτηριστικό is a neuter noun whose nominative and accusative singular are also the same in form

So although you do not see a visible change here, these nouns are being used as objects of the verb.

Greek often omits the article when talking about something in a more general, categorical, or list-like way.

So:

• Έλεγξαν διάτρηση can suggest they checked for perforation / a perforation
• Έλεγξαν χαρακτηριστικό ασφαλείας can suggest they checked a security feature / the presence of a security feature

If you added articles, the sentence could sound more specific:

• Έλεγξαν όχι μόνο τη διάτρηση αλλά και το χαρακτηριστικό ασφαλείας.

That version would sound more like:

• they checked not only the perforation but also the security feature

Both can be possible depending on context, but article omission is common in compact or technical-style phrasing.

διάτρηση is a feminine noun.

Basic dictionary form:

• η διάτρηση

It can mean things like:

• perforation
• puncture
• drilling
• piercing

The exact English word depends on context.

In this sentence, it is the first thing that was checked.

Because ασφαλείας is in the genitive case, and Greek often uses the genitive where English uses of.

So:

• χαρακτηριστικό ασφαλείας literally means
• feature of security

Natural English:

• security feature
• safety feature (depending on context)

This is a very common Greek pattern:

• noun + genitive noun
• X of Y

Here:

• χαρακτηριστικό = feature / characteristic
• ασφαλείας = of security / of safety

ασφαλείας is the genitive singular of ασφάλεια.

The noun is:

• η ασφάλεια = safety, security

Its genitive singular is:

• της ασφάλειας

In the phrase χαρακτηριστικό ασφαλείας, the article is omitted, but the noun stays in the genitive:

• ασφαλείας = of security / of safety

Here it is a noun, not an adjective.

As a noun, το χαρακτηριστικό means:

• feature
• characteristic

So:

• χαρακτηριστικό ασφαλείας = security feature

This can be confusing because forms related to χαρακτηριστικός also exist as adjectives, but in your sentence χαρακτηριστικό is clearly being used as a noun.

The aorist Έλεγξαν presents the action as a completed whole.

So it means:

• they checked rather than
• they are checking or
• they used to check

If you had a present form such as ελέγχουν, that would mean:

• they check
• they are checking

So Έλεγξαν tells you this was a completed checking action.

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

Your sentence:

• Έλεγξαν όχι μόνο διάτρηση αλλά και χαρακτηριστικό ασφαλείας.

is perfectly understandable.

But Greek can move things around for emphasis. For example:

• Όχι μόνο διάτρηση αλλά και χαρακτηριστικό ασφαλείας έλεγξαν.

That would put more focus on the things checked.

Still, the pattern όχι μόνο ... αλλά και ... usually stays together because it works as a set expression.

Yes, it is complete, but it has a somewhat concise, informational, or possibly technical/report-style feel.

Why it may feel compact:

• no explicit subject (they is understood from Έλεγξαν)
• no articles before the nouns
• short noun phrases

A fuller version could be:

• Έλεγξαν όχι μόνο τη διάτρηση αλλά και το χαρακτηριστικό ασφαλείας.

That may sound a little more explicit to learners, but the original is still valid Greek.

The stressed syllables are:

• Έ-λεγ-ξαν
• ο-ΧΙ
• ΜΟ-νο
• δι-Α-τρη-ση
• αλ-ΛΑ
• ΚΑΙ
• χα-ρα-κτη-ρι-ΣΤΙ-κό
• α-σφα-ΛΕΙ-ας

A rough pronunciation guide:

• Έλεγξαν ≈ EH-leg-ksan
• διάτρηση ≈ thee-A-tree-see
• χαρακτηριστικό ≈ ha-rak-tee-ree-STEE-ko
• ασφαλείας ≈ a-sfa-LEE-as

The written accents show which syllable gets the stress.