Translation:The people asked whether the laws are just.
The -u can be more than just a simple imperative, especially when the verb has a subject. Apart from a request or command, it can also express will, desire, purpose, etc. For example:
Adamo faru tion = Adamo should do it / Let Adamo do it
Oni ne manĝu piedpilkojn = One should not eat soccer balls
Mi iru hejmen = I should go home / have to go home
Ni manĝu = Let us eat
Nia amo daŭru eterne = May our love last eternally
Li piedpremu Lego-briketon! = May he step on a Lego brick!
More on this here: https://adventuresinesperanto.wordpress.com/tag/volitive/
Oh, then I misinterpreted what you meant by "Estu is more imperative than anything." Sorry.
But anyway, I don't think "estu" is a pure imperative in the case of "La homoj demandis ĉu la leĝoj estu justaj." I think it's closer to "should be" or "have to be". In English "whether" does not usually work with an imperative.
Depending on what one is trying to ask, yes, one could. Duo is asking us to think of laws in the present tense; but somewhere else the question may well be about the justness of past laws (estis). One could also ponder on the justness of future laws (estos), but again, that is not what Duo wants us to translate here. One can also wonder on whether laws should be just, or if injustice is inherent in the question of laws; that would be estus.
I hope that this helps.
I am not talking about the expressive part of the language so much as its grammar. What I mean is that, in some languages like Spanish, it is grammatically wrong to say e.g. 'I thought you are 20', you are forced to say 'I thought you were 20', because your main clause with the verb 'thought' is already in a past tense. In this example the verb 'demanded' is in the past, but then 'are' is in the present, so it seems to not use tense concordance. I am asking though, because there were a couple of sentences from a previous unit that seemed to obey tense concordance, but maybe they were talking about a past thing like you said? Btw thanks a lot for the help I appreciate it! (and sorry for the long explanation)
Ne problemo! I answered the question which I thought you were asking, and, being human, I erred a bit. English also seems to have a similar quirk to the one you are identifying as being a Spanish thing. But, since I grew up with that, I don't normally think about it until it is called to my attention. The sample sentence above is what people in the past were asking.
Esperanto tends to be a bit more literal than other languages. So, to use your example, Mi kredas, ke vi havas 20 jarojn, would be: "I think that you are 20 (years old)." The verbs all agree in time. Mi kredis, ke vi havis 20 jarojn = "I thought that you were 20(yo)." Again, the verbs all agree in time. One could say "I thought that you are 20" but one might have to bend a few customs, or one would need to be real clear about the timing. If the two verbs we are discussing are in the same time period, they have to agree.
This still allows for "I think that you will be…" or "I think that you were…" (Mi kredas, ke vi estos/havos kaj Mi kredas, ke vi estis/havis) I don't know the term "tense concordance" but it does seem to be the same thing that we are talking about.
All of this typing and I just realized that I used kredas which actually = believe not think. But that is the way that I learned to say this type of sentence. You may plug in pensi if you'd prefer, they are both understood equally.
And it's late, and I'm tired and I'll probably edit this thing again, in the morning. (If I can find it.)
Wow, thanks for the long reply! Yeah, English does this as well, though Spanish is what comes first in my mind because it's a little more strict about it. (e.g. I heard you study German would be wrong in Spanish, you can only say I heard you were studying German even if the person's still studying it now). So according to what you say in the example "Mi kredis, ke vi havis 20 jarojn.", which is exactly what I was thinking, should we report the sentence "La homoj demandis ĉu la leĝoj estas justaj." as wrong? Shouldn't it be "La homoj demandis ĉu la leĝoj estis justaj."? Or maybe I'm missing something cause it happens to be real late here too (or early.. it's almost 6am) and I'm awfully sleepy as well :P We'll see tomorrow! ^^
That would really depend upon the context. If people in the past were asking the question then it is possible to get just this construct. But yeah, I think that I'm going to have a short, sweet, e-mail barrage with some of the Duo staff here.
Further details if/when I find out what they were thinking.