Why does "den" need to be used to mean "the ant" if the "the" is already implied from the "-en" at the end of maur?
Because it's needed in front of an adjective. This also means that it can translate to both "the little ant" and "that little ant".
Why do I hear her say "Lili Marlene"?
Can i also use liten in place of lille?
"Liten" would be used with the indefinite form, "en liten maur", but not with the definite form.