Ο υπάλληλος είπε ότι το έγγραφο θα είναι έγκυρο μόνο αν δεν είναι ληγμένο και αν έχει σωστή σφραγίδα.

Here ότι means that and introduces reported speech:

Ο υπάλληλος είπε ότι...
= The employee said that...

It is different from ό,τι, which means whatever or anything that.

So:

• ότι = that
• ό,τι = whatever

In this sentence, ότι is the only correct form.

ο is the masculine singular definite article, so ο υπάλληλος means the employee with masculine grammatical marking.

A few useful points:

• ο = masculine singular the
• η = feminine singular the
• το = neuter singular the

So:

• ο υπάλληλος = the male employee / the employee marked as masculine
• η υπάλληλος = the female employee

The noun υπάλληλος can be used for either a man or a woman, and the article usually tells you which one is meant.

είπε is the aorist form of λέω, and here it means said.

Greek often uses the aorist for a single completed action in the past. In this sentence, the act of speaking is treated as one completed event:

• λέει = he/she says, is saying
• είπε = he/she said

So Ο υπάλληλος είπε... means the employee said it at some point in the past.

Because Greek usually keeps the future marker θα when the reported statement is future from the speaker’s point of view.

So:

• είπε ότι θα είναι έγκυρο
  literally = he said that it will be valid

In natural English, this is often translated as he said that it would be valid, but Greek does not backshift tenses as much as English does.

This is very normal Greek:

• Είπε ότι θα έρθει = He said that he would come
• Είπε ότι θα είναι έγκυρο = He said that it would be valid

Because έγγραφο is a neuter singular noun, and the adjectives describing it must agree with it in gender, number, and case.

Here:

• το έγγραφο = the document
• έγκυρο = valid
• ληγμένο = expired

All three are neuter singular, so they match.

This agreement is a basic rule in Greek:

• masculine noun → masculine adjective
• feminine noun → feminine adjective
• neuter noun → neuter adjective

That is why Greek says:

• το έγγραφο είναι έγκυρο
• το έγγραφο δεν είναι ληγμένο

Because σφραγίδα is a feminine noun, so the adjective must also be feminine.

• σωστός = masculine
• σωστή = feminine
• σωστό = neuter

Since σφραγίδα is feminine, Greek uses:

• σωστή σφραγίδα = correct / proper stamp or seal

This is the same agreement rule as with έγκυρο έγγραφο, just with a different gender.

μόνο αν means only if.

It introduces a condition that must be satisfied. So the sentence says the document will be valid only if certain things are true.

That is stronger than plain if:

• αν = if
• μόνο αν = only if

So the meaning is not just if this happens, it will be valid.
It is it will be valid only in that case.

The second αν is repeated to make the two conditions clear and parallel:

• if it is not expired
• and if it has the proper stamp

Greek can sometimes omit the second αν, but repeating it is very natural and often clearer, especially in more careful or formal language.

So both of these are possible in principle:

• μόνο αν δεν είναι ληγμένο και αν έχει σωστή σφραγίδα
• μόνο αν δεν είναι ληγμένο και έχει σωστή σφραγίδα

The version with the repeated αν makes the structure more explicit.

Because δεν is the normal negation for finite verbs in statements like this.

So:

• είναι = it is
• δεν είναι = it is not

Even after αν, Greek still uses δεν here:

• αν δεν είναι ληγμένο = if it is not expired

Learners often wonder about μη(ν), but μη(ν) is used in different environments, especially with να, commands, or prohibitions. In this sentence, δεν is the correct negation.

Yes, they look like present tense forms, and with είμαι and έχω they are exactly the forms Greek uses here.

A helpful way to think about it is this:

• after αν, Greek uses the form needed for a condition
• with some verbs, that form looks exactly like the present indicative

So:

• αν είναι = if it is
• αν έχει = if it has

There is no να after αν.

With other verbs, you may see forms that make the conditional pattern more obvious, for example:

• αν έρθει = if he/she comes
• αν γράψει = if he/she writes

So the structure in your sentence is completely normal.

Greek word order is more flexible than English word order, but it is not completely free.

This sentence could be rearranged for emphasis, for example:

• Το έγγραφο θα είναι έγκυρο μόνο αν δεν είναι ληγμένο και αν έχει σωστή σφραγίδα.
• Μόνο αν δεν είναι ληγμένο και αν έχει σωστή σφραγίδα, το έγγραφο θα είναι έγκυρο.

The basic meaning stays the same, but the emphasis changes.

Still, some things usually stay together:

• article + noun: το έγγραφο
• adjective + noun: σωστή σφραγίδα

So Greek is flexible, but not random.

Yes, in this sentence those are the best meanings:

• έγκυρο = valid
• ληγμένο = expired

Together they describe the legal or official status of the document:

• έγκυρο έγγραφο = a valid document
• ληγμένο έγγραφο = an expired document

So the idea is that a document can be officially acceptable only if it has not expired and has the proper stamp or seal.