I know about the theoretical differences in pronounciation, but speaking it`s not that obvious anymore; and if I have to write down after dictation... there`s my problem. How do you know what goes for the [i:]?
Since ι, η , υ, ει, οι, and υι are all identically pronounced as /i/, there's no way to tell from the sound of the word which of these is used -- you just have to learn the spelling.
Well it's not as bad as it seems.
Endings are standard and then, when you know what one word means and how it is spelled you can sort of easily guess how others with the same stem are going to be spelled.