Ο κιθαρίστας έχει κιθάρα.

Ο is the masculine singular definite article in the nominative case.
Here it means the, so Ο κιθαρίστας = the guitarist.

It matches κιθαρίστας because that noun is:

• masculine
• singular
• the subject of the sentence

So Greek articles change depending on gender, number, and case.

κιθαρίστας is a masculine singular noun and it is the subject of the sentence.

In this sentence:

• Ο κιθαρίστας = the subject
• έχει = the verb
• κιθάρα = the object

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

έχει is the 3rd person singular present tense of έχω = to have.

So:

• έχω = I have
• έχεις = you have
• έχει = he/she/it has

Here it agrees with ο κιθαρίστας, which is singular, so έχει means has.

Greek usually does not use an indefinite article the way English does.

So English a guitar is often just:

• κιθάρα

That means:

• ο κιθαρίστας έχει κιθάρα = the guitarist has a guitar

Greek commonly leaves the noun without an article when English would use a/an.

Because the sentence means a guitar, not the guitar.

Compare:

• Ο κιθαρίστας έχει κιθάρα. = The guitarist has a guitar.
• Ο κιθαρίστας έχει την κιθάρα. = The guitarist has the guitar.

So adding την would make it definite.

It is in the accusative singular, because it is the direct object of έχει.

The guitarist is doing the action of having, and κιθάρα is the thing he has.

A useful point: for many feminine nouns like κιθάρα, the nominative singular and accusative singular look the same:

• nominative: η κιθάρα
• accusative: την κιθάρα / κιθάρα

So the form of the noun itself does not change here, even though the case does.

A simple English-friendly pronunciation is:

o kee-tha-REE-stas E-chee kee-THA-ra

A few notes:

• κι sounds like kee
• θ sounds like th in think
• χ in έχει is a Greek sound not found exactly in English; before ι/e it sounds somewhat like a soft hissy h, often approximated as hee or khee
• Stress goes where the accent marks are:
  • κιθαρίστας
  • έχει
  • κιθάρα

Greek uses accent marks to show which syllable is stressed.

In this sentence:

• κιθαρίστας
• έχει
• κιθάρα

The article Ο is a one-syllable word, and monosyllabic words usually do not take an accent mark.

So the accents are mainly there to guide stress.

Greek word order is more flexible than English word order.

Ο κιθαρίστας έχει κιθάρα is the most neutral way to say it.

But Greek can change the order for emphasis, for example:

• Κιθάρα έχει ο κιθαρίστας.
  This puts emphasis on guitar.
• Έχει κιθάρα ο κιθαρίστας.
  Also possible, depending on context.

Even though word order can move around, the basic neutral pattern here is:

• subject + verb + object

In everyday Greek, κιθαρίστας is masculine. A common feminine form is:

• η κιθαρίστρια

So:

• Η κιθαρίστρια έχει κιθάρα. = The female guitarist has a guitar.

Notice how both the article and the noun change:

• ο κιθαρίστας = the male guitarist
• η κιθαρίστρια = the female guitarist

In standard Greek, that would usually sound unnatural here.

Greek normally uses the definite article with nouns like professions or identifiable people when they are the subject:

• Ο κιθαρίστας έχει κιθάρα.

Leaving out the article is not the normal equivalent of English guitarist has a guitar.
So for this sentence, keep Ο.

You can keep the same word order and use question intonation in speech:

• Ο κιθαρίστας έχει κιθάρα; = Does the guitarist have a guitar?

In writing, Greek uses the semicolon (;) as the question mark:

• Ο κιθαρίστας έχει κιθάρα;

That is one of the first punctuation differences English speakers usually notice.

Add δεν before the verb:

• Ο κιθαρίστας δεν έχει κιθάρα.
  = The guitarist does not have a guitar.

So:

• έχει = has
• δεν έχει = does not have

This is the normal way to negate a present-tense verb in Greek.