Ο συμπαίκτης μου γελάει.

Ο is the masculine singular definite article, meaning the.

In this sentence, ο συμπαίκτης μου means my teammate, but Greek normally still uses the article, so literally it looks like the teammate of mine.

Here, ο tells you that:

• the noun is masculine
• it is singular
• it is the subject of the sentence

That is just the normal Greek pattern. With possessive words like μου (my), Greek usually keeps the definite article:

• ο φίλος μου = my friend
• η αδερφή μου = my sister
• το σπίτι μου = my house

So ο συμπαίκτης μου is the standard way to say my teammate.

Μου means my, but grammatically it is really a weak genitive form, closer to of me.

So:

• ο συμπαίκτης μου = literally the teammate of me
• natural English: my teammate

In Greek, this little possessive word usually comes after the noun:

• η μητέρα μου = my mother
• ο δάσκαλός μου = my teacher

Because Greek and English organize possession differently.

English says:

• my teammate

Greek usually says:

• the teammate my = ο συμπαίκτης μου

This is one of the most common structural differences English speakers notice early on.

In this sentence, συμπαίκτης is a masculine noun, and the article ο matches that.

Very often, συμπαίκτης refers to a male teammate. The common feminine form is συμπαίκτρια.

So:

• ο συμπαίκτης μου = my male teammate
• η συμπαίκτριά μου = my female teammate

Greek nouns have grammatical gender, and the article has to agree with the noun.

A good approximation is:

sim-PEK-tees

A few useful points:

• συμ sounds like sim
• αι sounds like e
• the stress is on παί

So the stressed syllable is PEK.

A good approximation is:

yeh-LA-ee

The stress is on λά.

You may also hear it spoken a bit smoothly in fast speech, but for learning purposes it helps to think of it as:

• γε-λά-ει

Γελάει is the 3rd person singular present form of the verb γελάω / γελώ, meaning to laugh.

So:

• γελάω = I laugh / I am laughing
• γελάς = you laugh
• γελάει = he/she/it laughs or is laughing

Since the subject is ο συμπαίκτης μου, the verb has to be 3rd person singular.

Yes. In Greek, the present tense often covers both ideas:

• he laughs
• he is laughing

The exact meaning depends on context.

So Ο συμπαίκτης μου γελάει can mean either:

• My teammate laughs
• My teammate is laughing

In many real situations, it will be understood as is laughing.

Greek often leaves out subject pronouns because the verb form already tells you the person and number.

So instead of saying the equivalent of He is laughing, Greek can simply say:

• γελάει = he/she/it laughs / is laughing

In this sentence, the subject is already stated as ο συμπαίκτης μου, so there is no need to add αυτός (he).

Yes. Greek word order is more flexible than English.

The neutral order here is:

• Ο συμπαίκτης μου γελάει.

But you could also hear:

• Γελάει ο συμπαίκτης μου.

That still means the same basic thing: My teammate is laughing.
The difference is more about emphasis, rhythm, or context than basic meaning.

The accent marks show which syllable gets the stress.

• συμπαίκτης → stress on παί
• γελάει → stress on λά

Stress matters in Greek pronunciation, and sometimes it can help distinguish forms. So it is a good habit to learn Greek words together with their accent marks, not without them.

Both are used.

You may see:

• γελάει
• γελά

Both can mean he/she is laughing or he/she laughs.

For learners, γελάει is often easier at first because the ending is very visible, but γελά is also completely normal in modern Greek.