"ypsilon and hymn"
Translation:υ και ύμνος
η ι υ ει οι all used to be pronounced differently in Ancient Greek, which is why they have different spellings.
In Modern Greek, they're all pronounced identically as /i/, but Greek still uses historical spellings.
Like in English with "ee" versus "ea", which used to be distinct, but not "meet" and "meat" sound identical to almost all speakers, and you just have to learn which word is spelled how.
There are no rules of the kind "if it comes after the letter ... then it's spelled ...".