Κάθε ταξιδιώτης πρέπει να έχει μαζί του το διαβατήριό του.

In Greek, κάθε means each / every and normally takes a singular noun.

So:

• κάθε ταξιδιώτης = every traveler
• not κάθε ταξιδιώτες

This is similar to English every traveler, not every travelers.

Πρέπει means must / has to / should, depending on context. It is followed by να plus a verb form.

So:

• πρέπει να έχει = must have

This is a very common Greek pattern:

• πρέπει να πάω = I must go
• πρέπει να φύγουμε = we must leave

The word να introduces the verb that follows πρέπει.

Because the subject is κάθε ταξιδιώτης = every traveler, which is grammatically third person singular.

So the verb must also be third person singular:

• έχω = I have
• έχεις = you have
• έχει = he/she/it has
• έχουν = they have

Even though every traveler refers to people in general, it is still singular in grammar.

Μαζί means with, together with, or on/with one’s person depending on context.

Here έχει μαζί του means have with him / carry with him.

So μαζί του does not just mean physical togetherness in an abstract sense. In this sentence it means something like:

• carry it on him
• have it with him

This is a very natural Greek expression.

Because Greek commonly uses μαζί + weak pronoun in expressions like this:

• μαζί μου = with me
• μαζί σου = with you
• μαζί του = with him
• μαζί της = with her

Using με αυτόν would sound different and is not the normal phrasing here.

So for carry with him, Greek naturally says:

• έχει μαζί του

They do two different jobs.

1. μαζί του = with him
2. το διαβατήριό του = his passport

So the first του refers to the person the passport is with, and the second του shows possession.

In English, we might say:

• Every traveler must have his passport with him

Notice English also uses two forms:

• his passport
• with him

Greek is doing the same thing.

Yes, the basic word is διαβατήριο.

But when an enclitic like του follows, Greek often adds a written accent to show the correct stress:

• το διαβατήριο
• το διαβατήριό του

This extra accent helps show that the stress pattern is preserved before the following weak word του.

So διαβατήριό του is not a different noun; it is the same noun written with the accent needed in that context.

It is in the accusative case because it is the direct object of έχει.

The structure is:

• subject: Κάθε ταξιδιώτης
• verb: πρέπει να έχει
• object: το διαβατήριό του

The article το and the noun διαβατήριο are neuter singular, and in this sentence the accusative looks the same as the nominative:

• nominative: το διαβατήριο
• accusative: το διαβατήριο

Yes, in some contexts Greek could omit it if the meaning is obvious. But το διαβατήριό του is clearer and more natural if you want to emphasize that each traveler must have his own passport.

Without του, the sentence could still be understood, but it is slightly less explicit.

So:

• το διαβατήριό του = his passport / his own passport

This is a very normal choice in Greek.

Grammatically, ταξιδιώτης is a masculine noun. In some contexts Greek uses the masculine form generically, like English used to use he in general statements.

If you specifically meant a female traveler, you would normally say:

• η ταξιδιώτισσα
• μαζί της
• το διαβατήριό της

So this sentence can be understood as a general rule stated with the masculine form, but if you wanted to refer specifically to women, the forms would change.

The meaning stays the same if the word order changes a little. Greek word order is more flexible than English.

For example, these are all possible:

• Κάθε ταξιδιώτης πρέπει να έχει μαζί του το διαβατήριό του.
• Κάθε ταξιδιώτης πρέπει να έχει το διαβατήριό του μαζί του.

The first version is very natural. The second is also natural and may put slightly more focus on the passport.

So the order is not completely fixed, but the original sentence is a standard and idiomatic way to say it.