You can write with the big letters but you also can write with the small ones. You are right that there are more diferences on the greek small letters and the english letters. Βut like we use the big ones we use the smalls, with the same meaning. If we want to write the letter Β-β (in greek) we write b (the small one of B in English) That is we have in mind the big letters, that the most of them are exactly like the greek ones, but we write the smalls. And we can undenstand them because the spelling is the same.
for example: elpizo na katalabes - ελπίζω να κατάλαβες
ELPIZO NA KATALABES - ΕΛΠΙΖΟ ΝΑ ΚΑΤΑΛΑΒΕΣ
it means: i hope that you undenstand.
and I really hope that you unterstand!!
bye bye
Joanna from Cyprus