Salve, Terpsikeraunos!
I was wondering, in the style of making a comment underneath say an instagram post, how would you comment "so beautiful!' in Latin?
My first thought would be to address as vocative plural (pulchrissimae/i/a) but then ran into the problem of what gender to select .
Gratias tibi for helping out a former latin student who is crashing out in trying to grasp the grammar of a language i am starting to forget.
salve! using the superlative is a good idea. i think you'd want to use the accusative of exclamation (corresponding to the gender and number of whatever you're commenting on), unless you are directly addressing the subject of the photo, which would, as you said, require the vocative.
for example, if whatever it is happens to be a feminine noun, pulcherrimam! or you could use the nominative with adverbial quam, like quam pulchra [est]!

















