Ο αριθμός διαβατηρίου είναι σωστός.

Because Greek often uses the genitive after another noun to show a relationship like of in English.

So:

• ο αριθμός = the number
• διαβατηρίου = of passport / passport

Together, ο αριθμός διαβατηρίου means the passport number or more literally the number of the passport.

The basic dictionary form is το διαβατήριο.
The genitive singular is του διαβατηρίου.

In this sentence, the article του is omitted, which is common in fixed expressions and formal phrases like field labels.

Because σωστός agrees with ο αριθμός, which is a masculine singular noun.

Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Here:

• ο αριθμός = masculine, singular, nominative
• so σωστός must also be masculine, singular, nominative

That is why you get:

• σωστός (masculine)
• not σωστή (feminine)
• not σωστό (neuter)

Even though διαβατηρίου is neuter, the adjective is describing αριθμός, not διαβατήριο.

Because this is a normal Greek sentence pattern:

• subject + είναι + adjective

So:

• Ο αριθμός διαβατηρίου = the subject
• είναι = is
• σωστός = correct

This is the same basic structure as in English:
The passport number is correct.

In Greek, adjective position can change in some contexts, but after είναι, this is a very standard predicate adjective structure.

Because ο is the masculine singular definite article, meaning the.

• ο αριθμός = the number

Greek uses definite articles very often, including in many places where English also uses the.

Here it sounds natural because we are talking about a specific number, such as a value entered in a form.

The full genitive form would be του διαβατηρίου = of the passport.

So a fuller version would be:

• Ο αριθμός του διαβατηρίου είναι σωστός.

This is also correct.

But Greek often drops the article in this kind of formal noun phrase, especially in administrative or technical language:

• αριθμός διαβατηρίου = passport number
• αριθμός τηλεφώνου = telephone number
• άδεια οδήγησης = driving license

So the version without του is very natural, especially in forms, notices, and official wording.

Yes. That sentence is also correct.

There is only a small difference in feel:

• Ο αριθμός διαβατηρίου είναι σωστός sounds a bit more like a formal label or standard category: passport number
• Ο αριθμός του διαβατηρίου είναι σωστός sounds a bit more explicit: the number of the passport

Both can mean the same thing in practice.

Yes, in a normal complete sentence it is.

είναι is the present tense of to be for he/she/it is and also they are.

Here it means is:

• Ο αριθμός διαβατηρίου είναι σωστός.
• The passport number is correct.

Greek does sometimes omit είναι in headlines, notes, or very informal contexts, but in a standard sentence like this, you should include it.

It is in the nominative singular masculine.

That is because after είναι, predicate adjectives usually stay in the nominative and agree with the subject.

So:

• Ο αριθμός = nominative singular masculine
• σωστός = nominative singular masculine

This is different from some languages where the adjective might take another case, but in Greek it stays nominative here.

Yes, Greek word order is more flexible than English, but the basic version here is the most neutral:

• Ο αριθμός διαβατηρίου είναι σωστός.

You could change the order for emphasis, for example:

• Σωστός είναι ο αριθμός διαβατηρίου.

That sounds more marked, as if emphasizing correct.

For everyday neutral usage, the original order is best.

A rough pronunciation guide is:

o arith-MOS dhiava-tee-REE-ou EE-ne so-STOS

A few notes:

• θ sounds like th in think
• δ sounds like th in this
• the stress is important:
  • αριθμός
  • διαβατηρίου
  • είναι
  • σωστός

If you want to sound natural, try to keep the stressed syllables clear.

Greek does not have a true indefinite article exactly like English a/an.

It can use:

• ένας (masculine)
• μία / μια (feminine)
• ένα (neuter)

but these often mean one as well, and they are not always used where English would use a/an.

In this sentence, the definite article ο is used because the meaning is the passport number, not a passport number.

Yes, σωστός is a very common and natural word for correct/right.

In this context, it means that the passport number is valid as entered, not mistaken.

Depending on context, Greek can also use words like:

• ορθός = correct, proper, more formal
• έγκυρος = valid

But for a typical form or confirmation message, σωστός is very natural.

The dictionary forms are:

• ο αριθμός = number
• το διαβατήριο = passport
• είμαι = to be
• σωστός, σωστή, σωστό = correct

In the sentence, these appear as:

• ο αριθμός — same as the dictionary form
• διαβατηρίου — genitive singular of διαβατήριο
• είναι — present tense form of είμαι
• σωστός — masculine singular form of the adjective