I learnt Serbo-Croat before Greek and I have been struck by how much more help Serbo-Croat
is than English. This is not so much with the vocab ( though even here
there are quite a few more words that seem to have a Greek origin than in English).
It is more that things like turns of phrase often seem to have been taken from Greek.
This seems to me to go far beyond a retention of Indo-European forms such as the retention of
cases. My hunch is that it is due to Byzantine missionaries. Slovenia being further north may have
had less influence - but I really don't know Slovenian at all.