Ο γιατρός προετοιμάζει την εγχείρηση.

Because ο is the masculine singular nominative form of the definite article in Modern Greek.

Here, γιατρός is the subject of the sentence, so it appears in the nominative case:

• ο γιατρός = the doctor as the subject

Greek articles change for gender, number, and case, so you cannot use just one form of the everywhere.

Yes. Γιατρός can be used for both a male and a female doctor. The article usually shows the gender:

• ο γιατρός = the male doctor
• η γιατρός = the female doctor

So in your sentence, ο γιατρός tells you the doctor is male.

Because εγχείρηση is the direct object of the verb προετοιμάζει.

In Greek, direct objects usually go in the accusative case, so the feminine singular article changes:

• η εγχείρηση = the surgery/operation as a subject
• την εγχείρηση = the surgery/operation as an object

So:

• Ο γιατρός = subject
• την εγχείρηση = object

Because with this type of feminine noun, the singular nominative and singular accusative forms are often identical in the noun itself. The case difference is shown mainly by the article:

• η εγχείρηση
• την εγχείρηση

So the noun does not change here, but the article does.

This is normal in Greek and happens with many feminine nouns.

Because the next word, εγχείρηση, begins with a vowel.

In Modern Greek, the final -ν in words like τον and την is kept before:

• vowels
• and certain consonants

So:

• την εγχείρηση is correct, not τη εγχείρηση

That final -ν helps the phrase sound smoother.

It is the present tense.

In Greek, the present tense can often correspond to either:

• prepares
• is preparing

The exact English translation depends on context.

So Ο γιατρός προετοιμάζει την εγχείρηση could mean:

• The doctor prepares the operation
• The doctor is preparing the operation

In natural English, is preparing may sound more likely in many contexts, but Greek itself uses the same present form.

The dictionary form is προετοιμάζω, which means to prepare.

Προετοιμάζει is the third-person singular present form:

• προετοιμάζω = I prepare
• προετοιμάζει = he/she/it prepares, is preparing

So the ending tells you the subject is:

• he
• she
• or it

In this sentence, the subject is ο γιατρός, so it means the doctor prepares / is preparing.

Because Greek does not form the present continuous the way English does.

English uses:

• is preparing

Greek just uses the present tense verb:

• προετοιμάζει

So Greek does not need an extra word equivalent to is in this sentence.

Greek does have είναι = to be, but it is not used here to make a present continuous form.

Greek is more flexible than English.

1. The subject can sometimes be omitted because the verb ending already shows the person and number:

• Προετοιμάζει την εγχείρηση.
  = He/She is preparing the operation, if the context makes the subject clear

2. Word order can change for emphasis or style:

• Ο γιατρός προετοιμάζει την εγχείρηση. = neutral/basic order
• Την εγχείρηση προετοιμάζει ο γιατρός. = more emphasis on the operation

Your sentence uses the most straightforward, neutral order: subject + verb + object.

A rough pronunciation is:

o ya-TROS pro-e-ti-MA-zi tin en-HI-ri-si

A few helpful notes:

• γιατρός → stress on the last syllable: -τρός
• προετοιμάζει → stress on -μά-
• εγχείρηση → stress on -χεί-

A more Greek-like approximation:

• Ο = o
• γιατρός = ya-TROS
• προετοιμάζει = pro-eh-tee-MAH-zee
• την = teen
• εγχείρηση = roughly eng-HEE-ree-see

The exact sounds are a bit different from English, especially γ and χ, but this gets you close.

Both can relate to preparing, but προετοιμάζω is a bit more explicitly prepare beforehand / make ready, and it can sound slightly more formal or deliberate.

So:

• ετοιμάζει = prepares, gets ready
• προετοιμάζει = prepares, makes preparations for

In a medical or formal context, προετοιμάζει την εγχείρηση sounds very natural.

It usually means operation or surgical operation, and in many contexts it matches English surgery.

So depending on context, you may see it translated as:

• the operation
• the surgery
• the surgical procedure

The exact best English choice depends on the sentence around it, but the Greek word clearly refers to a medical operation.