"Is it his dog?"
Translation:Ist es sein Hund?
I used to find this confusing. A teacher once explained this to me with help of arithmetic!
The verb 'to be' is unique. For every other verb there is 'action' and the direct object 'receives' that action. Ich liebe meinen Hund. Sie sehen den Zug, ich öffnete die Tür. Dog, train and door are all in accusative case here.
BUT. With verb 'to be' there is no doing or done unto. You are simply describing a situation and both sides of the verb are equal. Like a sum, both sides are the same. 2 x 5 =10. 50 = 30 +20. 10 + 10 =20.
Das ist der Himmel. Ich bin dein Freund. Er ist sein Bruder. No activity in these three sentences , just statements. So, the sky, friend and brother are in the subject /nominative form. Does this help?
You cannot be sure it is a dog at all you're referring to here. That's why the subject of the interrogative sentence is "es". It can refer to any thing in general. If you hear the sound of an animal, you cannot be sure as to whether it is a dog at all. It might also be his hyena (die Hyäne), to which you would refer as "sie" to. So use a generic "es" for the "it".
seiner would be feminine genitive singular, feminine dative singular, or genitive plural.
Possessive determiners such as sein take similar endings to ein or kein -- thus no ending in masculine nominative singular.
sein Hund "his dog" but der Name seiner Katze (the name of his cat), for example, just as it would be ein Hund and der Name einer Katze.
seine Katze is "his cat" in the nominative case (e.g. as the subject of a verb) or the accusative case (e.g. as the direct object of a verb).
seiner Katze is "(of) his cat" in the genitive case of "(to) his cat" in the dative case.
Like the difference between die Katze and der Katze.
Because possessive pronouns and indefinite articles - the "-ein" words - don't have the "-er" suffix for masculine nominative.
If there something else in the sentence to take the "-er" ending, that word would have it.
"Ist es sein schwarzer Hund?" and "Ist es der schwarze Hund?"
"his dog" is usually sein Hund, using the masculine noun Hund which is used as the generic word for "dog" when the dog's gender is not relevant.
If the dog is specifically female and you want to point this out, you can use the feminine noun Hündin, in which case sein takes the ending -e and you have seine Hündin.
seine Hund (with feminine seine and masculine Hund) is never correct.
Unfortunately, Duo is not particularly clever about what to correct if two words don't match each other like that.
Hund is a masculine (mannelijk) word, so you need the masculine form sein in front of it.
sein is also used before neuter (onzijdig) words such as Pferd: sein Pferd = his horse.
seine would be used before feminine (vrouwelijke) or plural words: seine Katze (his cat); seine Tiere (his animals).