Learn Hardcore Greek: Όχι μόνο ελέγχουν δήλωση απώλειας αλλά χρειάζονται και υπεύθυνη δήλωση. - They not only check a declaration of loss but also need a solemn declaration.

Breakdown of Όχι μόνο ελέγχουν δήλωση απώλειας αλλά χρειάζονται και υπεύθυνη δήλωση.

What does Όχι μόνο ... αλλά ... και mean here?

It is the Greek pattern for not only ... but also ....

So the sentence is built like this:

That means: Not only do they check a loss declaration, but they also require a sworn statement.

Why is there no word for they in the sentence?

Because Greek often leaves subject pronouns out when the verb ending already shows who the subject is.

Here:

So Greek does not need an explicit αυτοί for they unless the speaker wants extra emphasis or contrast.

This is very normal in Greek.

Why does χρειάζονται look like a passive or middle form if the meaning is active?