A problem with accusative case
In the sentence" Der Prasident ist mein Bruder" surely it should be "meinen" as it is in accusative but I am told it is wrong, why?
I think "mein Bruder" in this case (sein verb) is a predicate nominative, not accusative.
XaviTuX is right. When using the verb 'sein' the noun is always in nominative, this also applies for 'heißen'. That's why you write stuff like:
Er ist ein Hund.
Mein Bruder ist ein Mann.
Der Präsident ist mein Bruder.
German is a very structured language and one of the things that you know you'll run into, for every rule, are exceptions heh
Look at it this way: The president an the brother are the same person. There must be a very good reason to give them different cases. Well: not here, so if one is Nominative because it is object, the other should be Nominativ also. Not scientific, but workable.
Specifically it's because they are both the subject. You use accusative when the thing that the verb is acting upon is the direct object. But with 'sein', as it's quite literally 'to be', the two things are the same thing and so both are the subject.