Modern Greek vs Ancient Greek

They’re the same language, not two languages

Greek has been written and spoken continuously for roughly 3,000 years, making it one of the longest-documented languages in the world. Modern Greek descends directly from Ancient Greek through a continuous chain of change, the same way Modern English descends from Old English, or Modern Italian from Latin. Linguists treat them as different historical stages of one language, not as separate languages the way French and Spanish are separate. That said, the gap is real: a fluent Modern Greek speaker cannot simply pick up Homer or Plato and read it fluently without dedicated study, any more than a modern English speaker can read Beowulf in the original.

What actually changed

• Grammar simplified. Ancient Greek had five noun cases and a dual number (a special form for exactly two of something); Modern Greek kept four cases and dropped the dual entirely. Verb conjugation lost some of its ancient complexity too, though it’s still far more inflected than English.
• Pronunciation shifted substantially. Ancient Greek had a pitch-accent system and vowel distinctions Modern Greek no longer makes. Several Ancient Greek vowels and diphthongs that were once pronounced differently all collapsed into the same modern “i” sound — which is exactly why η, ι, υ, ει, and οι all sound identical today despite being spelled differently.
• Vocabulary evolved, but less than you’d think. A huge number of everyday Modern Greek words are directly recognizable from their Ancient Greek roots — “νερό” (water), “άνθρωπος” (human being), and “καλός” (good) have all stayed close to their ancient forms for millennia.
• The script barely changed at all. The Greek alphabet used today is essentially the same alphabet used in antiquity — a rare case of a script surviving basically unchanged across three millennia.

The 20th-century detour: Katharevousa

For much of the 19th and 20th centuries, Greece had two competing written standards: Katharevousa (“purifying”), an artificially archaized form designed to look closer to Ancient Greek, and Demotic (“of the people”), the language people actually spoke. Katharevousa was the official language of government and education for decades. Demotic Greek — the everyday spoken language — won out and became the sole official standard in 1976. Modern Greek today is Demotic. This history is why some very old signage, church usage, and legal boilerplate still look noticeably more formal and archaic than everyday spoken and written Greek.