The difficulty is that English use of "shall" varies a lot with regions and generations. It used to be taught that the future simple tense was conjugated: I shall (go) - you will (go) - he/she/it will (go) - we shall (go) you[plural] will (go) - they will (go), meaning a simple act happening in the future; but reversing the "shall" & "will" to: I will (go) - you shall (go) - he/she/it shall (go) - we will (go) - you[plural[ shall (go) - they shall (go) added an imperative tone, meaning an act with definite intention or compulsion happening in the future. Nowadays, the "will" form is generally used as a straight future for all persons of the verb, but sometimes there is an element of "Willpower" too, e.g. "I will go and see the boss tomorrow to sort this problem out, whether he likes it or not!" The whole thing can also be further confused with English conditionals like "should" & "would" . . .which would take another page! I think there is an overlap here with how "sollen" can be used, but without context it's, as always with Duo - impossible to be sure which is best. Keep trying though!
From the conjugation table I learned that sollen -- shall, are to; sollten -- should, ought to. but from what I just practiced on Duo I also found that Sie sollen schlafen. -- They should sleep. (They shall sleep is also accepted) Sie sollten schlafen. -- They should sleep. It's a little confusing to me, since it looks like "should" can be a translation for both "sollen" and sollten", with little to no suggestion of tense. As a non-native English speaker, I wonder if you would say "They shall sleep at 10 pm (assuming that it's 8 pm right now)" and "They should sleep now (when it is about the time for them to sleep)"? I admit that in spoken language I seldom bother to distinguish that.
There's actually a reason why that's incorrect. While in English should often does mean ought to, in German there are essentially two versions of should: one that's like the English ought to, and one that implies necessity rather than a mere suggestion. Sollten implies that it's just a suggestion, and it's the version that can be translated to ought to. Sollen, the one used in this instance, is the version that more implies necessity. That's why ought to wasn't accepted. :)
"She" will conjugate its verb differently: "Sie spielt/geht/springt/denkt/soll [irregular]/ist [irregular]" vs. "Sie spielen/gehen/springen/denken/sollen/sind."
"They" and "you" conjugate the same, but "you" is always capitalized: "Spielen/sollen/etc. Sie?" vs. "Spielen/sollen sie?" Note that in speech, "they" and "you" will of course sound the same, so you would only know from context.
If "sie" is the subject, as it is in this sentence, the verb will conjugate differently:
- "Sie soll schlafen" = "She should sleep"
- "Sie sollen schlafen" = "They should sleep"
If "sie" happens to be a direct object, you can't tell and would have to infer which is meant by context:
- "Ich soll sie finden" is ambiguous
So, I said "they are to sleep" and was marked wrong. My thinking is, "supposed" isn't strictly necessary in English. Just curious if this is worth reporting as an alternate translation, or if there's a specific reason this is wrong, e.g. if there is a different way to say "they are to sleep" in German.