Duolingo second translation is "Bist du zu der Insel geschwommen?" for sentence "Did they swim to the island?". This is incorrect..."du" is not "they"
Yes, just the same. And here it shows "Sie sind zu der Insel geschwommen". I think it is a technical problem, cause there wasn't such an answer in the options.
Hi, why is Insel in dative form here? I thought for a sentence with implied motion "to the island" like this, it would be the same as "gehen ins Kino", aka accusative?
'Zu' always takes the dative case.