Το μοντέλο είναι καινούριο.

Το is the neuter singular definite article, so it means the.

Greek articles change for:

• gender: masculine, feminine, neuter
• number: singular, plural
• case

Here, μοντέλο is a neuter singular noun, so the article is το.

So:

• το μοντέλο = the model

Greek uses the definite article very often, and in this sentence it is completely natural and expected.

In Greek, every noun has a grammatical gender. Μοντέλο is a neuter noun.

That is why it goes with:

• το for the
• a neuter adjective form, here καινούριο

So the grammar is:

• το μοντέλο = neuter singular noun phrase
• καινούριο = neuter singular adjective

Even if a word refers to something that is not literally genderless, its grammatical gender still has to be followed in Greek.

Because it is agreeing with μοντέλο, which is neuter singular.

Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

The basic forms of this adjective are roughly:

• masculine: καινούριος
• feminine: καινούρια
• neuter: καινούριο

Since μοντέλο is neuter singular, Greek uses the neuter singular adjective:

• Το μοντέλο είναι καινούριο.

Είναι means is here.

It is the present-tense form of to be:

• είμαι = I am
• είσαι = you are
• είναι = he/she/it is, and also they are

So in this sentence:

• Το μοντέλο είναι καινούριο.
• literally: The model is new.

This is a very common Greek sentence pattern:

article + noun + είναι + adjective

Because καινούριο is being used as a predicate adjective after είναι.

In English, this is the same difference as:

• the new model → adjective directly before the noun
• the model is new → adjective after is

Greek works similarly:

• το καινούριο μοντέλο = the new model
• το μοντέλο είναι καινούριο = the model is new

If you added an article before καινούριο, you would be changing the structure and probably the meaning.

Yes, but that is not the same sentence structure.

Compare:

• Το μοντέλο είναι καινούριο.
  = The model is new.

• Το καινούριο μοντέλο
  = The new model

So:

• in your original sentence, καινούριο is after είναι and describes the subject
• in το καινούριο μοντέλο, the adjective is directly attached to the noun

Both are correct, but they mean slightly different things in context.

No, Greek word order is more flexible than English word order.

The sentence:

• Το μοντέλο είναι καινούριο.

is the most neutral, straightforward way to say it.

But Greek can move things around for emphasis, for example:

• Καινούριο είναι το μοντέλο.

That would sound more like:

• It’s the model that is new or
• The model is new, that’s the point

So the original order is the best one for a learner to use as the default neutral pattern.

Yes. You could say:

• Το μοντέλο είναι νέο.

This also means The model is new.

A useful rough distinction is:

• καινούριο often feels more like brand-new / new to use
• νέο can mean new, but sometimes also new, recent, modern, another new one

In many everyday situations, both are possible.
For learners, it is good to recognize both words.

A simple pronunciation guide is:

to mon-DE-lo EE-ne ke-NOO-rio

Stress falls on:

• μοντέλο → -τέ-
• είναι → εί-
• καινούριο → -νού-

A few helpful notes:

• το sounds like toh
• ει in είναι sounds like ee
• αι in καινούριο sounds like eh in modern Greek pronunciation

So if you say it smoothly, it sounds roughly like:

to mon-DE-lo EE-ne ke-NOO-rio

In normal neutral Greek, that would sound unnatural here.

Greek usually wants the article with a noun like this:

• Το μοντέλο είναι καινούριο.

Leaving out the article can happen in special contexts, headlines, labels, or certain set expressions, but it is not the normal way to say The model is new.

So for everyday Greek, keep the article:

• το μοντέλο