If "cxiu" already means "everyone," why does it need ths plural "j"?
For instance, in Spanish it is very common to say Agradezco a todos y cada uno, which means I thank all and everyone. It is a bit redundant, but it could be a reflection of a reality if you first said “thank you” to the group and then went to them, one by one, and did the same individually.
I guess danki ĉiujn is a more economical way to thank people than danki ĉiun, which multiplies the effort by the number of people in the group.
I'm not the expert here, but there seems to a slight difference between cxiu [every(one)] and cxiuj [all]. Extending the phrase, we can have cxiu homo [every person] and cxiuj homoj [all people]. Depending on the context, there could be a difference in meaning.
In this Duolingo question though, I'd say that both would be OK and would more or less carry the same meaning. They just chose cxiuj for this example, and therefore the translation has "all" instead of "everyone".
The answer is yes - -is covers all past tense.
You've posted several things about Esperanto over the last several days that are simply wrong. If you want to learn Esperanto, you should be using this time to learn Esperanto - and the way to do that is to listen to people who speak it.
Jane: I'm not sure I understand your question below. The translation certainly can be "who helped us". I said that -is can cover all kinds of past tense. This includes "who helped us" and "who had helped us." It's a feature of English, not Esperanto, that says you have to specify "helped" vs "had helped".
I reported it already... But to be quite sure (as I'm not a native English speaker). I wrote: We thank all those who helped us. And the corrector replied: You missed a word. We thank all those who has (underlined) helped us. I think that at least it should be "have", and reading the comments, I assume it is clear that "have helped" and "helped" are both correct. Ĉu ne?
No. ke is a different word. It just happens that ke and kiu could translate into the same word — that — in English. I consider English the exception in this case. Here is an example in Bulgarian:
ke = че
kiu = кой/който
And they are not interchangeable, just like in Esperanto.