sharp s and ss
I used to study German in highschool, and our teacher there thought us that writing "ss" is same as "ß". Is it correct, 'cause Duolingo doesn't recognize it?
It's "ss" after short vowels and "ß" after long vowels and diphthongs (ai, ei, eu, au, äu, ie). I highly recommend you don't just use "ss" everywhere as the distinction lets you know how to pronounce the preceding vowels.
@kcin and @gajaloc are right, there is a rule about the use of "ss" and "ß", they are different and are not freely exchangeable. It's not true, like @christian and @JacobWilliams said, that people is losing the use of ß. I live in Germany and everybody here, in official papers as in everyday writings, use it correctly. German keybords of course have the special ß character.
@scampaforche: I think you misunderstood what I said. I didn't say that the ß was disappearing. I was referring to the upper case ẞ, which is a relatively recent invention and hasn't really caught on. http://en.wikipedia.org/wiki/Capital_%E1%BA%9E Trust me, you definitely can't type this letter on a Germany keyboard. When writing in all caps, most people either use the normal lower case ß or they use SS.