Η αλλαγή εισιτηρίου χρειάζεται αριθμό κράτησης.

Because the subject is singular: Η αλλαγή εισιτηρίου.

Greek verbs agree with the subject, not with the object. Here:

• η αλλαγή = the change, singular
• χρειάζεται = needs / requires, 3rd person singular

So the sentence is built like:

The ticket change requires a booking number.

If the subject were plural, the verb would also be plural:

• Οι αλλαγές εισιτηρίων χρειάζονται αριθμό κράτησης.
  Ticket changes require a booking number.

Because Greek often uses the genitive after another noun to show a relationship like of in English, or like an English noun-noun combination.

So:

• αλλαγή = change
• εισιτηρίου = of ticket / ticket

Together, αλλαγή εισιτηρίου means:

• change of ticket
• more naturally in English, ticket change

The base form is:

• το εισιτήριο = the ticket
• του εισιτηρίου = of the ticket
• bare genitive form used here: εισιτηρίου

Because this phrase is being used in a general, classifying way.

Η αλλαγή εισιτηρίου sounds like ticket change as a type of process or service.

If you said η αλλαγή του εισιτηρίου, that would sound more like:

• the change of the ticket
• a specific, known ticket

So the version without the article is more compact and typical for notices, forms, or customer-service language.

Here χρειάζεται means needs or requires.

So the sentence is saying that a ticket change cannot be done unless you have a booking number.

A useful thing to know is that χρειάζεται is very common in instructions, notices, and service messages. Depending on context, it can feel like:

• needs
• requires
• is necessary

In this sentence, requires is often the most natural English equivalent.

Because it is the direct object of χρειάζεται.

The noun is:

• ο αριθμός = the number, nominative
• τον αριθμό = the number, accusative

In the sentence, the article is omitted, but the noun still stays in the accusative:

• αριθμό κράτησης

So Greek is still marking it as the thing that is needed.

For the same reason as εισιτηρίου: it depends on another noun.

Here:

• αριθμό = number
• κράτησης = of booking / of reservation

So αριθμό κράτησης means:

• booking number
• literally, number of booking/reservation

The base noun is:

• η κράτηση = booking / reservation
• της κράτησης = of the booking / reservation
• bare genitive used here: κράτησης

Because Greek often omits the indefinite article in concise, formal, or notice-like language.

So:

• χρειάζεται αριθμό κράτησης = requires a booking number

In more everyday speech, you could also hear:

• χρειάζεται έναν αριθμό κράτησης

That is also grammatical, but it sounds a little fuller and less like a system message or formal instruction.

So the version in your sentence is very natural for:

• websites
• customer-service notices
• ticketing systems
• forms and instructions

No. Greek word order is more flexible than English because case endings help show what each word is doing.

The sentence as given is neutral and clear:

• Η αλλαγή εισιτηρίου χρειάζεται αριθμό κράτησης.

But other orders are possible for emphasis. For example:

• Αριθμό κράτησης χρειάζεται η αλλαγή εισιτηρίου.

That puts more focus on booking number.

Greek can also rephrase the idea in a different structure, for example:

• Για την αλλαγή εισιτηρίου χρειάζεται αριθμός κράτησης.

That is closer to:

• For a ticket change, a booking number is needed.

Notice that in that version, αριθμός κράτησης becomes the subject, so the case changes.

Yes, but it sounds a bit formal and compact, like something you would read rather than casually say.

It fits contexts like:

• airline websites
• booking platforms
• FAQ pages
• automated customer-service messages

In everyday spoken Greek, someone might say something more direct, such as:

• Για να αλλάξεις το εισιτήριο, χρειάζεσαι τον αριθμό κράτησης.
  To change the ticket, you need the booking number.

So your sentence is natural, but it has an informational, service-style tone.

The stressed syllables are:

• η αλλαγή → stress on the last syllable
• εισιτηρίου → stress on ρί
• χρειάζεται → stress on ά
• αριθμό → stress on the last syllable
• κράτησης → stress on κρά

A rough pronunciation guide is:

ee al-la-YEE ee-see-tee-REE-oo hree-A-ze-teh a-ri-THMO KRA-tee-sis

A few notes:

• χρ in χρειάζεται does not sound exactly like English hr; the χ is a Greek sound with no perfect English equivalent.
• θ in αριθμό sounds like th in think.
• Greek stress matters, so it is worth paying attention to the accent marks.