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The term 'Ancient Greek' applies to the greek language mainly for the period 700bC to 100AD. Later the language, using the same alphabet is used in the 'byzantine era' mostly unchanged untill 1800.
Modern greek, the language established after 1800 utilises the same alphabet although there are changes in grammar, vocabulary and context.
For example, the 'dativ' case of ancient greek which exists in german and (i think) russian does not exist in modern greek although it is used in ancient greek.
In a few words i could say that a modern greek person of high school education cannot understand the context of an ancient greek text of 100BC (ptolemean era) more than 30%; where as a modern greek person of similar education would understand less than 3% of a script dated at 600BC.
