Ο γιος μου πηγαίνει στο λύκειο.

Ο is the masculine singular definite article, meaning the.

So Ο γιος means the son.

In Greek, it is very common to use the definite article with nouns, even in places where English might not use the. In this sentence, Ο γιος μου is the normal way to say my son.

γιος means son.

It is a masculine noun, which is why it takes the masculine article ο:

• ο γιος = the son

Greek nouns have grammatical gender, so articles and some other words must match the noun’s gender.

μου means my here.

In Greek, possessive words like my, your, his/her are often expressed with short forms that come after the noun:

• ο γιος μου = my son
• literally: the son my

This is completely normal Greek word order.

Some other examples:

• η μητέρα μου = my mother
• το σπίτι μου = my house

In standard Greek, the article is usually kept:

• ο γιος μου = my son

Leaving out the article is usually not the neutral, standard choice. So for a learner, it is best to think of the article + noun + possessive as the normal pattern:

• ο αδερφός μου = my brother
• η φίλη μου = my friend
• το παιδί μου = my child

πηγαίνω is the dictionary form, meaning I go.

In the sentence, the subject is ο γιος μου = my son, which is he, so the verb must be in the third person singular:

• πηγαίνω = I go
• πηγαίνεις = you go
• πηγαίνει = he/she/it goes

So:

• Ο γιος μου πηγαίνει... = My son goes...

It can mean either, depending on context.

Greek present tense often covers both:

• he goes
• he is going

In this sentence, with στο λύκειο, the most natural meaning is usually goes to high school or attends high school, especially if it is a general statement.

στο is a contraction of:

• σε = to / in / at
• το = the (neuter singular)

So:

• σε το → στο

That means:

• στο λύκειο = to the high school / at high school

This kind of contraction is very common in Greek:

• στο = σε + το
• στη = σε + τη(ν)
• στην = σε + την
• στον = σε + τον

λύκειο is a neuter noun, so it uses the neuter article το:

• το λύκειο = the high school

That is why σε το λύκειο becomes στο λύκειο.

Greek nouns belong to grammatical genders, and the gender does not always match anything logical in English. You simply learn each noun together with its article:

• ο γιος
• η κόρη
• το λύκειο

λύκειο refers to high school or the upper secondary school stage in Greece.

So πηγαίνει στο λύκειο usually means:

• he goes to high school
• he is in high school
• he attends high school

It is more specific than just school.

Yes. Greek often leaves out subject pronouns because the verb ending already shows the person.

So instead of saying:

• Αυτός πηγαίνει στο λύκειο = He goes to high school

Greek usually just says:

• Πηγαίνει στο λύκειο

In your sentence, the subject is stated explicitly as Ο γιος μου, so no pronoun is needed.

Yes, Greek word order is more flexible than English, although some orders sound more neutral than others.

The most neutral version here is:

• Ο γιος μου πηγαίνει στο λύκειο.

But other orders are possible for emphasis, for example:

• Στο λύκειο πηγαίνει ο γιος μου.
  This can emphasize to high school.

Still, as a learner, the original order is the safest and most natural one to use.

A simple pronunciation guide is:

O YEE-os moo pee-GHEE-nee sto LEE-kee-o

A few notes:

• γ before ι sounds somewhat like the y sound in yes
• γι in γιος sounds like yee
• ει in πηγαίνει is pronounced like ee
• the stress falls on:
  • γιος
  • πηγαίνει
  • λύκειο

So the stressed syllables are:

• γιος
• -γαί- in πηγαίνει
• λύ- in λύκειο

The accent mark shows which syllable is stressed.

For example:

• πηγαίνει → stress on γαί
• λύκειο → stress on λύ

This is important because stress is a real part of pronunciation in Greek. If you put the stress in the wrong place, you may sound unnatural or even say a different word.

So when learning vocabulary, it is very helpful to learn each word together with its accent.

It is grammatically definite because of the articles:

• ο γιος μου = my son
• στο λύκειο = to the high school

But in natural translation, English often makes it more general:

• My son goes to high school

So even though Greek literally has the, the natural English meaning is not necessarily My son goes to the high school. Context decides the best translation.