Ο γείτονας είναι πολύ φιλικός.

Ο is the masculine singular definite article in the nominative case. In this sentence, it means the.

So:

• ο γείτονας = the neighbor (male neighbor)

Greek uses articles a lot, and they must agree with the noun in gender, number, and case.

In this sentence, yes. Because the article is ο, the noun is understood as masculine, so it means the male neighbor.

A female neighbor is usually:

• η γειτόνισσα = the female neighbor

So:

• ο γείτονας = male neighbor
• η γειτόνισσα = female neighbor

The ending -ας is a very common ending for masculine nouns in Greek.

Here, γείτονας is in the nominative singular, because it is the subject of the sentence.

You will often see masculine nouns like:

• ο μαθητής = the student
• ο άντρας = the man
• ο γείτονας = the neighbor

The exact endings change depending on the noun pattern and case.

είναι is the verb to be, from είμαι.

Here it means is.

So:

• Ο γείτονας είναι φιλικός. = The neighbor is friendly.

A useful thing to know: είναι can also mean are in other contexts, because the same form is used for both he/she/it is and they are.

Examples:

• Αυτός είναι εδώ. = He is here.
• Αυτοί είναι εδώ. = They are here.

Greek often does not need subject pronouns like he, she, or it, especially when the subject is already clear.

Here, the subject is already stated:

• Ο γείτονας = the neighbor

So there is no need to add he.

This is very normal in Greek.

Here, πολύ means very.

It is modifying the adjective φιλικός:

• πολύ φιλικός = very friendly

So the structure is:

• είναι πολύ φιλικός = is very friendly

Because here πολύ is being used as an adverb, not as an adjective.

As an adverb, it means very, and adverbs do not agree with nouns.

So:

• πολύ φιλικός = very friendly
• πολύ καλή = very good
• πολύ όμορφο = very beautiful

In all of these, πολύ stays the same.

This can be confusing because πολύ can also appear as an adjective form in other contexts, but in this sentence it clearly means very.

Because Greek adjectives must agree with the noun they describe in:

• gender
• number
• case

Since γείτονας is masculine singular nominative, the adjective must also be masculine singular nominative:

• φιλικός = masculine
• φιλική = feminine
• φιλικό = neuter

Examples:

• Ο γείτονας είναι φιλικός. = The male neighbor is friendly.
• Η γειτόνισσα είναι φιλική. = The female neighbor is friendly.
• Το παιδί είναι φιλικό. = The child is friendly.

It often means friendly, but it can also have a sense like amicable or kind in manner, depending on context.

In everyday Greek, φιλικός is a normal and natural word for describing a person as friendly.

So in this sentence, it works exactly as you would expect:

• πολύ φιλικός = very friendly

Yes, Greek word order is more flexible than English, but not every order sounds equally natural.

The most natural order here is:

• Ο γείτονας είναι πολύ φιλικός.

You may also hear other orders for emphasis, but for a learner, this standard order is the safest one to use:

• subject + verb + adverb + adjective

So this sentence is a very good basic model.

A rough pronunciation is:

• γείτονας ≈ YEE-to-nas

A few notes:

• The stress is on the first syllable: ΓΕΙ
• The first letter γ before ει sounds softer than English hard g
• The ει is pronounced like ee

So the whole word is approximately:

• YEE-to-nas

A rough pronunciation is:

• είναι ≈ EE-ne

More carefully, it is usually pronounced in two syllables:

• EE-ne

The stress is on the first syllable.

Even though it is spelled with several vowels, in normal Modern Greek pronunciation it is quite compact.

A rough pronunciation is:

• fee-lee-KOS

The stress is on the last syllable:

• φι-λι-ΚΟΣ

That final stressed syllable is important.

The accent marks show which syllable is stressed.

In this sentence:

• γείτονας → stress on γεί
• είναι → stress on εί
• πολύ → stress on λύ
• φιλικός → stress on κός

Stress matters in Greek, both for pronunciation and for correct spelling.

It is in the nominative case, because it is the subject of the sentence.

That is why you have:

• ο γείτονας = the neighbor (subject)

If it were an object, the form would change:

• Βλέπω τον γείτονα. = I see the neighbor.

So in your sentence:

• ο γείτονας = nominative subject
• είναι = verb
• πολύ φιλικός = description of the subject

For normal Greek, that would not be the standard way to say The neighbor is very friendly.

You usually keep the article:

• Ο γείτονας είναι πολύ φιλικός.

Leaving it out would sound unusual here, or it could suggest a different kind of meaning or style.

So as a learner, you should definitely use the article in this sentence.