What is different between Koine Greek and Ancient/Classical Greek?

Question

What is different between Koine Greek and Ancient/Classical Greek?

Answer 1

I don't know specifics, exactly, but I do know that Koine greek was the catch-all term used for Greek that was mixed with the language of the region. So, for instance, Ptolemais probably spoke an Arabic-Greek hybrid and it was called Koine, while Baghdad spoke  a Persian-Greek hybrid and it was still called Koine.

Answer 2

Classical Greek was spoken around the 5th century BC. Although there were several dialects, the term generally refers to the variety spoken in and around Athens (Ionic / Attic dialect).

The koine ("common language") developed at the time of Alexander's conquests (3rd century onwards), and was sort of an international language used to facilitate communication among Greeks from different regions as well as foreigners.

The two varieties differ in a number of features, the most prominent being pronunciation: Classical Greek distinguishes between long and short vowels, and therefore has 7 vowels and several diphthongs (clusters of two vowels). By contrast, the vowel system of the koine only provides for 5 "pure" vowels and no diphthongs.

Another difference is that the koine tends to use word stress rather than intonation. This means that important syllables are pronounced louder than unimportant ones, much like in English. Classical Greek achieved the same effect by rising or lowering voice pitch.

Although the koine was probably simpler to pronounce than Classical Greek, it was rather harder to write. For one, the koine distinguishes between capital and minuscule letters, whereas in Classical Greek they used ALLCAPS (often without spaces between words!). In addition, koine words are written with a number of signs to indicate  intonation, aspiration or lack of aspiration and -in some cases- derivation. These were invented by grammarians to facilitate foreigners to read the words "properly" (i.e. according to the Classical norms) but were probably a burden, especially after these pronunciation features had become defunct.