"I am reading all evening."
Translation:Jeg leser hele kvelden.
I thought Kveld was 'evening' and kvelden was 'the evening' I answered 'Jeg leser hele kveld.' because it didn't ask for 'the evening' Is the definite article always needed here, for example 'Jeg arbeider hele uken'?
No, it would not make much sense in this sentence.
'Alle' - 'Everyone'
'Alt' - 'Everything'
'Hel' - 'Whole'
All of these can be used when talking about humans, animals, objects etc, but they're quite different in meaning and use.
'Alle' is used when describing that most or all out of a number of things/individuals is doing something/recieving something. Example: "Vi burde invitere alle" (We should invite everyone), "Alle dyrene er ute, bortsett fra hundene" (All the animals are outside, except the dogs)
Similarly with 'alt', except that its mostly used about things: "Har du alt du trenger?" (Do you have everything you need?), "Alt er klart" (Everything is ready)
'Hel/hele', however, is used when talking about a single thing or period of time. "Har du lest hele boka?" (Have you read the whole book?), "Jeg har en hel uke å gjøre oppgaven på" (I have a whole week to do the assignment).