"The river runs through the whole country."
Translation:Floden løber gennem hele landet.
"Hel" can take two meanings:
Here it is always "HELE + definite Noun"
Hele landet = all of the specific country.
Here we move the article to the front and conjugates "Hel" according to the definity and pluralty of the article, followed by the indefinite plural or singular noun.
Det hele land = The unbroken country.
De hele lande = The unbroken countries.
Et helt land = An unbroken country.
the latter can also take the meaning "entirety" as in "all of an non-specific country"