"I look at a screen all day" should be the favored translation, unless the meaning of "hela dagarna" is closer to "every day." Looking at this sentence in Swedish, I get the impression it means both all day and every day. If so, a good translation would be "I look at a screen all day every day." Could a native Swedish speaker please tell us the implications of this expression.
Apparently Swedish compresses as "hela dagarna" an experience of employees with expertise in computer work that they are working all day at the computer, but with emphasis that this experience repeats every day. English would expand the experience to state "I work at the computer all day" to express the primary idea, then add the emphasis, "every day," to state that this experience is repeated each day. "All days" is not an adequate translation.
- hela dagarna means "all day", in the sense of "all day, every day"
- hela dagen means "all day", either in the above sense or as in the entire day a specific day
Clearly, "all days" is not a very good translation, and I have removed that. This has implications for the reverse translation, and usually, we might argue that an unidiomatic English translation is better for teaching the Swedish sentence better - but in this case, the English translation is flat out wrong, so we'll just have to live with it. :)
I wrote "I watch a screen the whole day". It gives that sense of the infinitive even though it does not correlate exactly with the Swedish, but to translate the Swedish word for word would be incorrect in English. Anyway, I was marked as incorrect and "all days" was Duo's answer so I have reported it.
I disagree. Watch/watching sounds wrong when talking about a computer/phone/tablet screen since you would normally interact with it. Watching TV is entirely passive. Google backs me up: a search for "watch a screen all day"" gives 99 results; "look at a screen all day" gives 33,400
Both of the people above who asked for it also speak English natively, though. I think this may just be one of those cases where different natives have different ideas.
Either way, the default is still "look at", and Duolingo has finally implemented a feature to only show the best translations to users, so I see little harm in accepting it. :)