Translation:They are visitors from another star.
I think it should be "von" not "vom" since vom = von+dem and if you use vom, it inherently means "from the" (von dem), so einem/eine has no place...sorry if I said it wrong I am just at intermediate level and this grammar is just absolut schön. I wish English followed such rules.
Could.it be because of declensions https://en.m.wikipedia.org/wiki/German_declension#Definite_articles
That's not necessarily a distinction that translates, and the difference between "on" and "in" in English is often due to convention rather that physical properties. So there's no reason to assume that another language would use the same prepositions.
For example, you could be on a planet, not in it. You can be in Canada, not on it, even though in each case you'd be on the surface of it. You can be on the road and something can be in the street. I can live on Smith street, but 150 years ago in the US, I would have said that I live in Smith street. I can queue up in New York and be on line. I'd be physically standing on the line. I can go to California and be in line, even though there's no enclosure. There might be ropes, but there might not be.
These and many other examples show that "in" doesn't necessarily mean that something is inside of something, and "on" doesn't necessarily mean that something is on top of something else. If I say that I live on Smith Street, chances are that what I mean is that my house is adjacent to Smith Street but not on the street itself.
I think this article might help you: http://www.learn-german-smarter.com/learn-german-adjective-endings/
As far as I understood, you never have two cases declensions in a row, like 'eineM andereM', the second word, in this case an adjectiv, will have a weak ending, that doesn' t indicates the case. So in 'eineM andereN' , the "M" indicates the case, and the "N" is not indicating the case at all! Note that if the preceding word doesn't have a strong ending, then you would need to use 'andereM'. I hope I am right, and that I made it clear enough.
Der Stern is not an indirect object becase it is not being affected by the action of the verb; nothing is hapening to it directly or indirectly. Rather it is an object of a preposition and it's case is dictated by the preposition. "von einem anderen Stern" is in dative (reflected by the "m" in "einem") only because "von" takes the dative case.
Why is it "anderen" and not "anderem". Checking the word in wiktionary, this must be "mixed masculine singular dative form of anderer", is this correct? It might not be the missing word in the question, but it still is confusing, as I'm not taught strong/weak/mixed declensions yet
Ich muss mich leider einmal wieder beschweren, weil Sie von Duolingo die Woerter "Sie" und "sie" nicht richtig bewerten. "Sie sind …" kann entweder eine eher foermliche Anrede eines Bekannten oder Fremden sein, den man noch nicht per Du anspricht, oder es kann auch die Pluralform von mehreren Personen sein. Sie muessten eigentlich beide Uebersetzungen als richting markieren.
They are visitors from another star. (oder ebenfalls:) You are a visitor from another star.
Beide Saetze muessten meines Erachtens nach gleichmaessig ins Deutsche uebersetzt warden.
Not exactly, because it is not accusative, but dative here. "anderer" is declined like an adjective. And after an indefinite article you have to use the "mixed inflection" table.