"Where do they hear it?"
Translation:Họ nghe thấy nó ở đâu?
In general, those two can be interchangeable.
On the other hand, a stand-alone "nghe" would mean "to obey/to oblige to". "Nghe thấy" means "to hear".
They are the same. Just difference between lắng nghe ~ listen and nghe/nghe thấy ~ hear
Earlier I remember it being said that "ở đâu" and "đâu" were almost interchangeable. Is there a reason "ở" must be included in this sentence?
No it should not be interchangeable. Can you please point out where (in tips & note) so I can fix it?
"ở đâu" literally means "at where" and they are used together in most of the time when asking "where". Using a standalone "đâu" means a very different thing: you are denying a statement/a fact/a truth or anything proposed by someone else.
Found it! It's under Questions 1.
"Using where - đâu
Đâu is placed at the end of the sentence.
Đâu is often used with ở, which means at. Literally, ở đâu means at where."
The words "often used with" imply that it is not always used with it.
Yup, ở is often missed out in SPOKEN Vietnamese...like a contraction in English; I am becomes I'm.... Mẹ ở đâu becomes mẹ đâu.