Does anyone have a good way to remember when one uses "ß" vs. "ss"? I have seen the "ss" used in spelling elsewhere where Duo uses the "ß". A quick online investigation seems to indicate that "ss" is used only after short vowels. Is this true?
Yes, that is the way it is supposed to be after the last reform. It is a good thumb-rule provided you can differentiate between long and short vowels. 'ss' used to be used previously in old texts instead of "ß".