Translation:They have been playing together all day.
That seems about right ! Tack !
It the sentence "a past event that has present consequence" in the tips & notes that threw me out. It's the definition of "imparfait" in french (Ils jouaient ensemble) and not passé-composé (ils ont joué). But the latter must be the right one still :)
I take it this cannot be translated "they played" because they are still playing. That's what the English present perfect continuous used here would mean. "They have played together all day" would mean they may have stopped. English has three ways of denoting this past activity, but Swedish only two, so how do they overlap?
We don't have a continuous form so our present perfect covers both. You can translate the sentence either as they have been playing or as they have played, but not as they played because that would be de lekte. In De har lekt tillsammans hela dagen they may have stopped or they may still be playing, you can't tell.
In the same way, de lekte can be either 'they played' or 'they were playing'. And de hade lekt can be either 'they had played' or 'they had been playing'. So it's just that we don't really have a separate continuous form.
Let's take another example: hela (den) långa dagen. 1) Because of the pronoun hela, the adjective lång is conjugated långa. It is not common that one adjective is conjugated according to another adjective, but according to a pronoun, e.g. samma fina exempel. 2) I also think that the facultative pronoun den would take another place if hela wasn't a pronoun. You can't often see a combination adjective + pronoun + adjective + noun, can you?
At least in Swedish grammar books published in Finland, hela is always categorized as a pronoun. It would be very interesting to know what Svenska akademiens grammatik thinks of hela.
Alright. I'm not saying you're wrong, I'm just a bit surprised. :) In Svenska Akademiens Ordlista (link below), "hela" is an adjective or an adverb as far as I can see. http://www.svenskaakademien.se/svenska-spraket/svenska-akademiens-ordlista-saol/saol-13-pa-natet/sok-i-ordlistan
Hela can also be translated as the entire or whole sometimes. in any case the sentence clearly states that the playing has finished so my translation "played the entire day is correct. Have been playing is a progressive form and therefore indicates that they are still playing.