Definite and indefinite articles do not exist in Russian. "Они видят дверь" can mean both.
the pronunciation of the 'р' in 'дверь' sounds weird to me, why does it sound like a 'D'?
I think that because of the softening, which should make the "р" sounds less like a rolling "r", they might be just cutting short the normal "р" sound and that sound you hear is this cut.
Why did it correct me as "They can see the door"? I don't see any reason to translate it as "can see".
Why is the word "door" not conjugated to accusative since it is the object (or at least I think it is?) and it is feminine? :-)
why does видят sound exactly like видит? shouldn't it be closer to "vidyat"?
No they don't. Ь doesn't have its own sound. It palatalizes the preceding consonant.
Milan403977 I think you mean ы sounds similar to и. Ы kinda makes the ( i ) sound as in ( hit ). Compared to И, the ( ee ) sound like in ( dream ) and both the ( ee )+( y ) sound in ( sweetly )