Όχι μόνο ελέγχουν δήλωση απώλειας αλλά χρειάζονται και υπεύθυνη δήλωση.

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

• Όχι μόνο = not only
• αλλά = but
• και = also / too

So the sentence is built like this:

• Όχι μόνο ελέγχουν δήλωση απώλειας
• αλλά χρειάζονται και υπεύθυνη δήλωση

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

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

Here:

• ελέγχουν = they check
• χρειάζονται = they need / they require

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

This is very normal in Greek.

Because χρειάζομαι is one of those Greek verbs that has middle/passive endings but an active meaning.

So:

• χρειάζομαι = I need
• χρειάζονται = they need

In this sentence, it does not mean are needed. It means they need / they require.

A useful comparison:

• Χρειάζεται υπεύθυνη δήλωση. = A sworn statement is needed.
• Χρειάζονται υπεύθυνη δήλωση. = They need a sworn statement.

Since the sentence already has ελέγχουν with an implied they, the natural reading is that the same they also χρειάζονται the second document.

Greek can omit the indefinite article much more easily than English.

So instead of saying:

• μια δήλωση απώλειας
• μια υπεύθυνη δήλωση

Greek often just says:

• δήλωση απώλειας
• υπεύθυνη δήλωση

This is especially common in formal, bureaucratic, or list-like language, where the speaker is naming document types rather than emphasizing a / one specific item.

If you added μια, the meaning would still be similar, but it would sound a little more like a single document rather than just the general requirement.

Because δήλωση απώλειας literally means declaration of loss.

Here:

• δήλωση = declaration / statement
• απώλειας = of loss

The second noun is in the genitive to describe what kind of declaration it is.

This is a very common Greek pattern:

• πιστοποιητικό γέννησης = birth certificate
  literally certificate of birth
• άδεια οδήγησης = driving licence
  literally licence of driving
• δήλωση απώλειας = loss declaration
  literally declaration of loss

Because in Greek, και often goes right before the element that means also.

So here:

• αλλά χρειάζονται και υπεύθυνη δήλωση

means roughly:

• but they also need a sworn statement

The και is attached to υπεύθυνη δήλωση as the extra thing being added.

You will often learn the pattern as όχι μόνο ... αλλά και ..., and that is correct. But in real sentences, Greek word order is flexible, so other words can come between αλλά and και.

They are in the accusative, because they are the direct objects of the verbs:

• ελέγχουν δήλωση απώλειας = they check a loss declaration
• χρειάζονται υπεύθυνη δήλωση = they need a sworn statement

A point that can confuse learners: with many feminine singular nouns, the nominative and accusative look the same.

For example:

• η δήλωση = nominative
• τη δήλωση = accusative

Without the article, the form δήλωση looks the same, so you identify the case from the sentence structure and the verb.

υπεύθυνη is an adjective modifying δήλωση.

It agrees with δήλωση in:

• gender: feminine
• number: singular
• case: accusative singular here

So:

• δήλωση = feminine singular
• υπεύθυνη must match it

That is why you get υπεύθυνη δήλωση.

This phrase is also a fixed expression in Greek for an official kind of declaration, so learners often meet it as a set phrase.

Yes. Both ελέγχουν and χρειάζονται are in the present tense and describe a general, ongoing, or habitual situation.

So the sentence is not talking about one specific moment only. It sounds more like a general rule or procedure:

• they check
• they require

This is very common in instructions, regulations, and administrative language.