"Grandfather is not my father."
Translation:Großvater ist nicht mein Vater.
The placement of "nicht" can be difficult. The most useful idea that I have come across is this:
In short, "nicht" is always placed in front of the negated element. If the sentence as a whole is negated, this means that the verb is negated, but that has to always occupy the second position and cannot be moved further down. However, the verb is often split into a finite part (conjugated part) in second position and other things that are placed in the end of the sentence, which altogether has been simply named verb box. The "nicht" becomes a part of the verb and is placed in front of the verb box. Sometimes the verb box is empty, then "nicht" will be in end position.
In sentences of the structure "X is Y" (see Paralars1) Y is a predicative. In this case "to be" is only a copula verb that is completed by Y, which is part of the verb and placed in the verb box at the end of the sentence. In the example sentence the predicative is "mein Vater".
Ich sehe dich nicht. (empty verb box, "nicht" at the end)
I do not see you.
Ich sehe dich nicht an. (separable prefix "an" from verb "ansehen" in the verb box)
I do not look at you.
Ich habe dich nicht gesehen. (participle in the verb box)
I have not seen you.
Ich bin nicht dein Vater. (dein Vater is a predicative noun of verb "dein Vater sein")
I am not your father.
Ich bin nicht grün. (grün is a predicative adjective of the verb "grün sein")
I am not green.
I kann nicht schwimmen. (schwimmen is the infinitive full verb to modal verb können)
I can not swim.
Almost. In sentences of the structure „a is b.“ a is the subject in nominative, is is a copula verb that is completed with the predicative noun b also in nominative. Here, the predicative noun „dein Vater“ should be in nominative not accusative. Hence,
Luke, ich bin dein Vater.
In a (simple) declarative clause, when you negate the (inflected) verb:
Er redet. – Er redet nicht.
Sie mag ihn. – Sie mag ihn nicht.
Wir lesen das Buch. – Wir lesen es nicht.
No, the verb "sein" requires the nominative case. It is a copula verb (explanation on canoo.net: http://www.canoo.net/services/OnlineGrammar/Wort/Verb/VollHilfModal/Kopulaverb.html?lang=en)
Please also note that in German, nouns have to be written with a capital letter: Großvater ist nicht mein Vater.
No, not anytime. You should use the nominative for predicative nouns like in the example sentence. You mostly find predicative nouns in sentences of the structure "A is B." Here, A is the subject in nominative, "is" is the copula verb that is completed with the predicative noun "B" also in nominative. You could say the completed verb is "to be B". This means you have two nominatives and no accusative in this kind of sentence.
You have the same thing in English, but it often leads to confusion, because the case systems got almost lost in English. Almost means that there are some remnants in the English pronouns. I/me, she/her, he/him....
This is he and I on the photo.
While this is very correct, it sounds unusual to hear nominative pronouns as predicative nouns in nominative.
This is him and me on the photo. This is very common and acceptable in English. The object case pronouns sound more in line with the feeling of having an object case after the verb. In German with a more flexible word order and rich case system, the same would sound simply wrong. German insists on the nominative.
Das sind er und ich auf dem Bild. (never ihn or mich)
Can someone explain me, why the verb "to be" (seid), don't have the direct object in accusative case? Actually I'm always using the nominative case with the verb "to be", but never think that a negated sentence get it too. Just in case, there is other verbs like this one?
The grammatical concept you are looking for is "predicative noun". Predicative nouns are usually in nominative and they mostly but not exclusively occur with the verb "sein" in German and just as well with "to be" in English. However, since English doesn't have much of a distinction between nominative and object case, there is hardly any consequence for this. In German predicatives are not inflected (nouns in nominative, adjectives without inflected endings).
The difference is if the possessive (my/mein) is used as a determiner or as a pronoun. The same goes for English with the forms (my/mine).
A determiner is connected to a noun (in place of an article).
Großvater ist nicht mein Vater.
Grandfather is not my father.
A pronoun stands in place of a noun.
Meiner ist nicht mein Vater.
Mine is not my father. (strange example)
I missed the t off my ist, massive fail, obviously. I was a little distracted. But to any learners suffering distress from Duolingo's incredibly insensitive phrase, please accept my love. Duo does great language courses but in the real World (you know, Duo?) people suffer all sorts of dreadful complications in their lives. Just appalling, Duo.