What everyone here seems to be failing at explaining is that "NICHT" comes at the end of a sentence but BEFORE an adjective. In the case given here:
Großvater = noun ist = verb mein Vater = adjective
So it should be written: Noun --> Verb --> Negation --> Adj.
Großvater ist NIGHT mein Vater.
"mein Vater" is used as the adjective to describe the noun, which is "Großvater". That means we place our negation before "mein Vater."
Note that "mein Vater" is not an adjective; it is a noun (or, more accurately, a noun phrase). Because this sentence uses a linking (copulative/copular) verb, "mein Vater" is called the "subject complement". (Wikipedia has this list of copular verbs.) An adjective can be used with a linking verb (z.B., "Großvater ist alt."), but it is still called a "subject complement".
If the verb were an action verb, then "mein Vater" would be referenced as a "direct object": z.B., "Großvater sieht
mein meinen Vater nicht." ["Grandfather does not see my father."] Note how nicht moves to the end of the sentence here. With the sentence for this exercise, nicht is placed just before the element of the sentence it is negating. A very good discussion on the position of nicht can be found at YourDailyGerman.com.
In order to be Accusative, you need a subject AND an object. Meaning, we need TWO different entities.
Unfortunately, "mein Vater" is used as an adjective in this case. Compare it to, "Der Apfel ist nicht golden." 'nicht golden' is used to describe 'Apfel' as an adjective and the sentence is NOT accusative. If we apply the same to "Großvater ist nicht mein Vater," we see that 'nicht mein Vater' is used as the adjective to describe 'Großvater.' 'Großvater' is both the subject and object, so no accusative case is in play here.
I believe Duolingo provides all special characters right next to the text input area. Simply click on the special letter that you want to insert at the cursor location.
Otherwise, if you're on a Windows computer: Hold down the ''Alt'' key and type ''225'' on the number keypad. That produces a ''ß''.
On an Android device hold down the ''s'' key on the keyboard and wait until the special characters menu pops up. Select the ''ß''.
I use Windows 8 and I added a Spanish and German keyboard to my settings/preferences. Now I can push the windows key plus the space bar to cycle between keyboards of different languages, then "-" becomes "ß". Although I couldn't type it in this box without using the alt code given by zengator, I hate remembering alt codes.