ο

the definite article (ο, η, το) — goes before a known, specific thing and agrees with it in gender, number and case · το βιβλίο — книга (та самая, о которой речь)

used where English drops it too: with names, countries, days of the week and abstract nouns · η Ελλάδα, ο Γιάννης — Греция, Янис