Questions & Answers about Ο αντίπαλος παίζει καλά.
Ο is the masculine singular definite article. It works like the in English.
So ο αντίπαλος is a full noun phrase meaning the opponent. In Greek, the definite article is used very often, including in places where English might sometimes leave it out.
It is masculine, singular, nominative.
- ο = nominative masculine singular article
- αντίπαλος = nominative masculine singular noun
It is in the nominative because it is the subject of the sentence, the person doing the action.
Here it is being used as a noun.
But αντίπαλος can also function as an adjective in Greek, with the sense of opposing or rival depending on context. In this sentence, though, it clearly means opponent/rival, so it is acting as a noun.