Why wouldn't "zij" be correct? Every time I think I get the hang of this stressed/unstressed difference, I end up messing up. Basically, I get that it's ze because it's unstressed, but I don't know why.
If you are still on Duolingo, "zij" is now accepted as an answer.
Waarom is deze zin niet correct "Ik hoop dat komen ze"
This one sentence consists of two main clauses: "ik hoop dat" and "ze komen". In the main clause, the verb comes second, so it is 'ze komen' and not 'komen ze'.