Would there be any pronunciation difference between " Der Großvater ist.. " and " Der Großvater ißt..." ?
But if you're wondering if this sentence could also mean "...doesn't eat my father", remember that that sentence would require accusative case (...meinen Vater) and would cause nicht to go to the end. There may be some other sentences with pun potential, though.
Won't accept 'Grandad'??? I thought Granddad was the US spelling. It accepts 'mum' and 'mom', so why not poor old Grandad?
Because we are still talking about the grandfather and saying that he is not his father. So this makes the father subject too, because that is a reference to the grandfather.
Similar to "Es ist ein Apfel", "Das ist der Käse" etc.
Why is "Der Großvater ist nicht meinen Vater." not correct? Thanks in advance
Because when saying "nicht mein Vater" you are still talking about the subject, that is the "Großvater". As you are still talking about subject you can't use "meinen" because then it would mean that "Großvater" or "Nicht Mein Vater" is an object. Both are subjects and refer to the same person.