You can say, "en hel dag".
This might confuse you more, but anyway...
"Hel" is sometimes an adjective, and sometimes a determiner. A determiner is a word like "a", "the", "that", "this" that tells you which particular instantiation of noun you're referring to. "Hel", "halv" and "all" similarly tell you how much of that instantiation of a noun you're referring to.
In the Duo exercise above, you don't mean, "for a whole day", which would be using the word adjectivally. You mean, "for the whole of a day", which is using it as a determiner.
When used as a determiner, it's "hele". And this sort of word is also used before other determiners.
The whole of the sad day.
Hele den triste dagen.
You're right, I am confused. So as an adjective, it follows the hel/helt/hele rule. As an adverb, it is always in the neuter form. And as an determiner, it is always in the plural/definite form? But no, I don't really see how "for a whole day" is different than "for the whole of a day." Maybe I will understand it better when I am more advanced in my study. How would you amend this sentence (about the brannmann) to change "hel" into an adjective? And into an adverb?
So as an adjective, it follows the hel/helt/hele rule. As an adverb, it is always in the neuter form. And as an determiner, it is always in the plural/definite form?
But no, I don't really see how "for a whole day" is different than "for the whole of a day."
I'm not sure I can explain it much better, but consider:
In my job, you can be rostered on for whole days or half days. On Wednesday, I worked the whole of a whole day. But on Thursday, I could only work half of a whole day. I was rostered on for a whole day, and then I went home sick.
"You can be rostered on for whole days or half days" uses the words adjectivally. I could as easily have written "sad days" or "sun-filled days". I'm just describing a noun. However, when I say, "the whole of a whole day", the first whole, in Norwegian, doesn't attach a quality to a noun. Rather, it says how much of that noun I'm talking about.
Another example of an adjective use:
The ship was whole.
Skipet var helt.
So, when you say, "was whole", "whole" is an adjective. If you see, "a whole noun", "whole" is also an adjective. However, if you see "the whole noun", "whole" is sometimes an adjective, and sometimes a determiner. It depends on the meaning. Most often, it's a determiner.
How would you amend this sentence (about the brannmann) to change "hel" into an adjective? And into an adverb?
You can't really amend the sentence. The meaning is talking about the extent of the noun "day". You're not describing a quality that the day possesses.
i asked a similar question elsewhere: i am wondering whether Norway, too, has moved away from using patriarchal terms in the same way we in the US are trying to move away from them: congressperson, not congressman; firefighter, not fireman; mail carrier, not mailman; first-year, not freshman.
Kjønnsnøytralt språk, kjønnsinkluderende språk, inkluderende språk eller kjønnsnøytralitet er en form for språklig forskriftivisme som tar sikte på å eliminere (eller nøytralisere) referanse til kjønn i termer som beskriver mennesker. Dette kan innebære motløshet fra bruken av kjønnsspesifikke stillingsbetegnelser, for eksempel politimann / politikvinne, brannmann, flyvertinne, og uten tvil styreleder, til fordel for tilsvarende kjønnsnøytrale begreper som politibetjent, brannmann, flyvertinne og leder (eller stol). Andre kjønnsspesifikke termer, for eksempel skuespiller og skuespillerinne, kan erstattes av det opprinnelig mannlige begrepet (skuespiller brukt for begge kjønn).