The Three Genders

Every Greek noun has a fixed grammatical gender - masculine, feminine, or neuter - a property of the word, not of its meaning. You predict it from the noun’s nominative-singular ending: -ος / -ας / -ης (and borrowed -ές) are masculine, -α and -η are feminine, and -ο / -ι / -μα are neuter. The article ο / η / το does not tell you the gender - it agrees with the gender the ending already reveals. A few endings have exceptions (most importantly -μα is neuter, not feminine), so the article is still worth learning per word.