English is not a highly inflected language, and for that reason native English speakers often find noun cases confusing. In the sentences
The dog sees the cat.
The cat sees the dog.
we do not spell 'cat' and 'dog' differently. We understand which animal is the subject of the verb - which animal is doing the seeing - only via word order; in these examples, the subject comes first.
Greek, on the other hand, is a highly inflected language. Word order is flexible, and meaning comes from the use of different case endings. To read and understand a Greek sentence, you must be able to recognize the case form of each noun.
There are five noun cases in koine Greek: the nominative, genitive, dative, accusative and vocative. Today we will start our look at the nominative case.
1) The nominative case is used for the subject of a sentence.
Ο απόστολος βλέπει τον προφήτην.
The apostle sees the prophet.
'Ο απόστολος' is in the nominative case because 'the apostle' is the subject of the verb: he is doing the seeing. We have two clues indicating nominative: the 'ο' - the definite article in its masculine nominative singular form, and the '-ος' ending of 'απόστολος', also indicating masculine nominative singular.
If the sentence was reversed
Ο προφήτης βλέπει τον απόστολον.
The prophet sees the apostle.
the form of the nouns used would not be the same. Instead we would have 'τον' instead of 'ο' for the definite article, and 'απόστολον' instead of 'απόστολος' for 'apostle'.
What is the subject of this new sentence? 'Ο προφήτης', 'the prophet'. The word for 'prophet' is now in the nominative case instead of the word for 'apostle' (which is now in the accusative case; more on that later). Notice that although our first clue is the same (the definite article is always 'ο' for masculine nominative singular) the ending of the noun itself is different: '-ης' instead of '-ος'.
Why? (ask generations of Greek students, beating their heads against the nearest wall). It would certainly be easier if all masculine nouns ended in '-ος' in the nominative singular.
But they don't. The proper declension and pattern of endings must be identified for each word. Fortunately, after even a little reading in the New Testament you will become familiar with the endings of the most common nouns, and recognition of the proper forms will become automatic.
I promise. Next time we will move on to the second major use of the nominative case: the predicate nominative. In the meantime, χαίρετε!