Why would it not accept, "Is it their's?"
When you add מ- it means from
Fellow learner but I think "Is it their's?" would be "זה הן?". From them is "מהן".
"Is it theirs?" - זה שלהם/הן?
"Is it them?" - זה הם/הן?
"Is it from them?" - זה מהם/הן?
Or does "זה הן" mean "is it them?"?
"Is it theirs?" would be "זה שלהן?"
I wrote, "that's from them?" Isn't that acceptable?
Yes, it really should be. Report it.
I think Duo is quite strict about verb-preceding-noun in English questions.
עוד בעיות של זכר ונקבה
It didn't accept "This is from them?" Why?
Because it wasn't reported yet. It is correct.
Bicouse this is a question
The given answer is also a question.
theirs in English means from them, so it should be a correct translation, Or not?
Theirs and from them mean two different things, both in English and in Hebrew. Theirs is שלהם or שלהן and from them is מהם or מהן.
Yes of course. You are right, תודה