Yes you can see it's hard using genealogy sites with a relative called יצחק . What spelling? Then if they went to Eastern Europe it could turn into iecj or something... (this happened to me with Isaac, isaak, yitzak, Ісаак, Isaacas.... then you find out it was really a nickname & have to start all over... I realised you just leave EVERYTHING in Hebrew and let the search feature on the genealogy websites figure it out!)
So, what if I am not a native English speaker? The two sentences (the one listed above, and the one you suggested) might have a similar meaning, but that is not the exact translation, and therefore not accepted. Duolingo requires us to translate as close as possible to the original sentence, to get a better grasp of Hebrew. Both of you asked why it's not correct, and I explained why your answers are not accepted. There is a difference between "this is a/the boy" and "this boy". There is no "belong" in this sentence. I am trying to help, since I've finished the tree a long time ago and trying to give insight to people who struggle with things like these, but if you're not willing to accept help from a "non-native English speaker", don't worry. I won't bother next time.