Let’s see the following sentences:
(a) O Πέτρος θέλει γιατρό. “ Peter needs a doctor.”
(b) Ο Πέτρος είναι γιατρός. “ Peter is a doctor.”
Sentence a clearly involves two persons, Peter and the doctor. Peter is the subject and the doctor is the object. Unlike sentence a, sentence b involves only one person, Peter. What about the doctor? The doctor of b is not another person, it’s Peter’s profession. Technically, the word γιατρός of b is not an object but a predicate to the subject. It gives extra information about the subject. In this sense doctor works like an adjective, and Modern Greek adjectives must follow the case of the noun they modify (this is the technical term for such relations). This is why we have the nominative (γιατρός) in sentence b.
Note that a predicate may modify not only the subject but also the object. In that case, it will be in the accusative:
Τα παιδιά έκαναν τον Πέτρο ήρωα. (NOM. ο ήρωας) “The children made Peter a hero.”
You can also read: Subject-Verb agreement
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