"The rich man hides my letter."
Translation:Người đàn ông giàu giấu thư của tôi.
why does this particular sentence require Nguoi with dan ong but others don't?
Because of the adjective "rich". Without classifier, you are saying that all rich men hide your letter, which can be, grammatically a correct answer but does not sound right.
Will we be getting audio for these excerises eventually? We can't really learn to pronounce things other wise :(
=The rich man hides his letter (in your proposal, mình refers to the previously mentioned subject người đàn ông)