"A grandmother is learning."
Translation:Eine Großmutter lernt.
Assuming that it said "Eine Großmutter ist am Lernen":
Standard German doesn't have separate simple and progressive forms, so "Eine Großmutter lernt" can, depending on the context, be translated either as "A grandmother learns" or as "A grandmother is learning."
There is, however, a special progressive form used in colloquial speech in some regions: a form of "sein" (to be) + am + capitalised infinitive of the main verb: Sie ist am Lernen (She is learning.) Ich bin am Kochen (I am cooking.) Wir sind am Singen (We are singing.)
That's a tricky question.
1) In Switzerland, they don't use the letter "ß" at all, so everything written with "ß" in Germany is written with "ss" instead. The normal Swiss spelling of this word is "Grossmutter".
2) In Germany, an "ß" is used after long vowels such as the "o" in "Großmutter". If you have absolutely no access to special German characters such as "ß" for some reason, you are allowed to use "ss" instead. But that's not really the case here, as Duolingo provides you with all the characters you need.
I have an android phone and through the swipe app you can install other language keyboards. I can switch between two languages of my choice by pressing and holding the space bar so a menu comes up with languages I dowloaded. German has its own keys for ä, ö, ü, and best of all ß.