I cannot understand what is wrong with "on the page 8"? My instructor claims that it is "on page 8". It is a specific page to which I referring to on a particular book. What is wrong with "the" in front of "page 777"?
I'd say it's because, when referring to the page by it's number, the number is essentially the "name" of the page. In that sense, you wouldn't refer to someone named John as "The John".
Conversely, when it's "the 7th page", you're referring to a page at a specific position, rather than referring to it by "name" ("the guy over there", rather than "John").
Hopefully that's helpful to you.