Ο γιατρός ελέγχει τον εγκέφαλο.

Ο is the masculine singular definite article in the nominative case. In this sentence, it means the and goes with γιατρός.

So:

• ο γιατρός = the doctor

Greek articles change form depending on gender, number, and case, so you will not always see the same form for the.

Because τον εγκέφαλο is the direct object of the verb.

Greek shows the job of a noun by using case:

• ο εγκέφαλος = nominative, used for the subject
• τον εγκέφαλο = accusative, used for the direct object

In this sentence, the brain is being checked, so it is the object, not the subject.

This is a normal noun change for many masculine nouns ending in -ος.

For this pattern:

• nominative singular: ο εγκέφαλος
• accusative singular: τον εγκέφαλο

So the ending changes from -ος to -ο in the accusative singular.

Another common example:

• ο άνθρωπος
• τον άνθρωπο

You can tell from the case and the verb form.

• ο γιατρός is nominative, so it is the subject
• τον εγκέφαλο is accusative, so it is the object
• ελέγχει is third person singular, matching ο γιατρός

So Greek grammar tells you that the doctor is checking the brain.

Ελέγχει is the present tense, active voice, third person singular form of the verb ελέγχω.

That means it is the form used with:

• he
• she
• it
• or a singular noun like ο γιατρός

So:

• ελέγχω = I check
• ελέγχεις = you check
• ελέγχει = he/she/it checks

It can mean either, depending on context.

In Modern Greek, the simple present often covers both:

• checks
• is checking

So Ο γιατρός ελέγχει τον εγκέφαλο can mean:

• The doctor checks the brain
• The doctor is checking the brain

Usually the situation or surrounding context makes the meaning clear.

Because Greek often does not need subject pronouns when the subject is already clear.

Here, the subject is explicitly stated as ο γιατρός, so there is no need to add a pronoun like he.

Even when there is no noun subject, Greek often leaves the pronoun out because the verb ending already shows the person:

• ελέγχει already tells you it is he/she/it or a singular noun

Greek uses the definite article very often, more regularly than English does.

Here both nouns are definite:

• ο γιατρός = the doctor
• τον εγκέφαλο = the brain

In Greek, it is completely normal to include the article with both the subject and the object when both are specific.

Yes. Greek word order is more flexible than English word order because case endings help show who is doing what.

The sentence as written:

• Ο γιατρός ελέγχει τον εγκέφαλο

is the most neutral, straightforward order: subject + verb + object.

Other orders are possible for emphasis, but they may sound more marked or require additional structure. For a learner, this sentence is the best basic pattern to remember.

The accent mark shows which syllable is stressed.

So:

• γιατρός → stress on the last syllable
• ελέγχει → stress on the second syllable
• εγκέφαλο → stress on κέ

Stress matters in Greek pronunciation, so the accent marks are important and should not be ignored when learning new words.

The form γιατρός can be used for a doctor in general, but the article shows the gender in standard usage:

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

So in your sentence, ο γιατρός specifically means a male doctor.

The dictionary form is the nominative singular:

• ο εγκέφαλος

Greek dictionaries usually list nouns in that form. So if you look up εγκέφαλο, you would normally need to know that its base form is εγκέφαλος.