Two years later and I'm still confused. Curse these German cases... Yes, Sohn is masculine but, since it's Sohnes (Pl), why isn't it ihrer?
Is it actually because of the singularity of the determiner rather than the plurality of the noun (i.e. because 'her' refers to a single person)? Would 'I am the father of THEIR son' be 'Ich bin der Vater ihrer Sohnes'?
Sohnes is not plural; es ist Genitiv, Singularform. Sieh dir dies an.
The determiner and the noun will match in number, gender, and case. You cannot have a singular determiner (z.B., "der") with a plural noun (z.B, "Männer"). If you see "der Männer", you know that because the noun is plural, the determiner must be plural, thus it would be "die" (Nominativ oder Akkusativ), "den" (Dativ), oder "der" (Genitiv). In this way, you then--even without the rest of the sentence--know that "der Männer" must be Genitiv (the "owner" in a possessive situation: "die Mutter der Männer" ==> "the mother of the men").
https://deutsch.lingolia.com/en/grammar/pronouns/possessive-pronouns Idk why they don't just give us this chart at the start
It is, and should be accepted in a "translate German to English" exercise.
If you get it marked wrong again, check that you didn't have a "type what you hear" (i.e. type the German words in German) exercise rather than translation and that you have the newest version of the app if you're using the app; then post a screenshot.
I actually translated it as, "I am the father of their son." Not only is it possible to find application with a sperm donor and a Lesbian couple, but also, I could be the biological father of my son who is adopted. Also, in cases wherein a man's name is placed on a birth certificate, and later it is discovered that he was not the biological father; it applies there. One might also say that if a grandfather has been a father and cared for his grandson as if he were the father, he may say, "I'm the father of their son." (emphasizing his role as opposed to biological standing). I'm sure there's more but I think it was right!!!! :)