Here, "covered in" suggests a layer of dust sits on top of the items/furniture in the room.“Everything in the room was covered IN a fine film of dust.”
Here, "covered with" just means there were a lot of forest areas spread all over the island.“The whole island is covered WITH tropical forests.”?
If you have a question about the English language and would like to ask one of our many English teachers and language experts, please click the button below to let us know: