"Watching television is very interesting."
Translation:Xem tivi rất hay.
ing = participle / gerund. In this case it is the gerund use which is why đang does not work because it can only be used with verbs. Essentially it is the verb "to watch" that was made into the gerund (noun) "watching" English marks these with -ing. vietnamese seems to just assume you can figure it out.
I think the mouse dictionary (mouse on the dots) should reflect these remarks. The first translation given is đang xem and the solution with only xem is not given.
Why does xem and coi mean the same thing? They seem like such different words and not just a northern/southern "accent".
What is the purpose of the hover hints if they are not going to indicate syntactically correct "hints"? Just call them "randomly generated grammar grab bags". Much more helpful...