Παίζω εναντίον του γείτονα.

Because Greek often does not need a separate subject pronoun.

The verb παίζω already tells you the subject is I:

• παίζω = I play

So Εγώ παίζω εναντίον του γείτονα is possible, but εγώ is usually omitted unless you want emphasis, contrast, or clarity.

For example:

• Παίζω εναντίον του γείτονα. = I play/am playing against the neighbor.
• Εγώ παίζω εναντίον του γείτονα, όχι εσύ. = I’m the one playing against the neighbor, not you.

Here, παίζω means I play in the sense of:

• playing a game
• playing a sport
• taking part in a match or competition

With εναντίον (against), the meaning is clearly competitive:

• Παίζω εναντίον του γείτονα. = I play against the neighbor.

Depending on context, παίζω can also mean other kinds of play, such as playing music or acting playfully, but in this sentence it means compete/play against someone.

Εναντίον means against.

It is used to show opposition, whether physical, competitive, or figurative:

• παίζω εναντίον κάποιου = play against someone
• είμαι εναντίον αυτής της ιδέας = I am against this idea

In this sentence, it introduces the person you are competing against:

• εναντίον του γείτονα = against the neighbor

Because εναντίον requires the genitive case in Greek.

So:

• ο γείτονας = the neighbor (nominative, dictionary/basic form)
• του γείτονα = literally of the neighbor, but after εναντίον it means against the neighbor

This is a very common thing in Greek: certain prepositions or expressions require a specific case.

So the structure is:

• εναντίον + genitive
• εναντίον του γείτονα

Because Greek cases often do jobs that English expresses with prepositions.

In isolation:

• του γείτονα often means of the neighbor

But after εναντίον, the whole phrase means:

• εναντίον του γείτονα = against the neighbor

So you should translate the whole phrase, not each word separately.

This is very normal in Greek. The case form is grammatical, but the natural English meaning comes from the full expression.

The base form is γείτονας = neighbor.

This is a masculine noun. Its singular forms are:

• ο γείτονας = nominative
• του γείτονα = genitive
• τον γείτονα = accusative
• γείτονα! = vocative

In your sentence, the form is γείτονα, but because it comes after του and after εναντίον, it is functioning as genitive singular.

In this sentence, γείτονα comes from the masculine noun γείτονας.

If you want to say against the female neighbor, you would use the feminine form:

• η γειτόνισσα = the female neighbor
• εναντίον της γειτόνισσας = against the female neighbor

So:

• Παίζω εναντίον του γείτονα. = I play against the male neighbor.
• Παίζω εναντίον της γειτόνισσας. = I play against the female neighbor.

Παίζω is the present tense.

In Greek, the present tense can mean:

• I play
• I am playing
• sometimes even I do play, depending on context

So without more context, Παίζω εναντίον του γείτονα could mean:

• I play against the neighbor
• I’m playing against the neighbor

Greek often relies on context more than English does for this distinction.

Yes. Greek word order is more flexible than English word order.

The neutral order here is:

• Παίζω εναντίον του γείτονα.

But you may also hear:

• Εναντίον του γείτονα παίζω.
• Του γείτονα παίζω εναντίον. (less neutral, more marked)

The basic meaning stays the same, but the emphasis changes. For learners, the most natural version to use is:

• Παίζω εναντίον του γείτονα.

A simple pronunciation guide is:

• Παίζω ≈ PEH-zo
• εναντίον ≈ e-nan-DEE-on
• του ≈ too
• γείτονα ≈ YEE-to-na

A few useful notes:

• αι in παίζω sounds like e
• ει in γείτονα sounds like ee
• γ before ει sounds like a y sound in English, so γεί- sounds like yee-
• The stressed syllables are:
  • παίζω
  • εναντίον
  • γείτονα

In this sentence, the article του makes it specific:

• του γείτονα = the neighbor

Greek uses the article very often with specific nouns, more often than English in some situations.

If you mean a neighbor, you would usually say:

• εναντίον ενός γείτονα = against a neighbor

So:

• εναντίον του γείτονα = against the neighbor
• εναντίον ενός γείτονα = against a neighbor

Using εναντίον γείτονα by itself would sound unnatural in normal standard Greek.

It is most naturally understood as a game, sport, or competition, because of παίζω.

So the most likely meaning is something like:

• playing chess against the neighbor
• playing football against the neighbor
• playing a game against the neighbor

It would not normally mean a serious fight or argument. For that, Greek would use a different verb, depending on the situation.

So yes, the sentence is mainly competitive/play-related, not hostile in a literal fighting sense.

Yes, but usually in a clear context where a game or match is already understood.

For example:

• Σήμερα παίζω εναντίον του γείτονα στο τάβλι.
  Today I’m playing against the neighbor in backgammon.
• Αύριο παίζουμε εναντίον των γειτόνων.
  Tomorrow we’re playing against the neighbors.

On its own, Παίζω εναντίον του γείτονα is grammatically correct and natural, but in real life speakers often add more context if needed.