"Trade between the countries of the European Union is important."
Translation:Handel mellan den Europeiska Unionens länder är viktigt.
The sentence isn't talking about the trade being important, but rather about it being important to have trade. And such constructions always take the neuter form in Swedish. The other alternative would be "handeln [...] är viktig", in which case the trade is emphasised.
The sentence is still here, and "det är viktigt med" is still accepted.
Just curious, but why was the sentence [supposed to be] deleted? It's slightly complex in structure, but it's at the end of the tree, and it's something I might expect to read in the news, even simplified news like 8sidor.
Primarily, it has so large error rates that I want to teach the construction in different ways instead. If too many people get it wrong, we're either teaching it too early or using too difficult examples.
But it's a combination of several reasons, really. I imagine most native Swedes would use slightly different constructions for this sentiment, so it's not even the most idiomatic option.
It also features several separately difficult-to-translate things together, which is rarely a good thing in a course for beginners. In a B2 course (for instance), it'd be perfectly appropriate.
It's worse than that. You tend to use of as a sort of catch-all where we don't want to use av - remember "a cup of coffee = en kopp kaffe" which throws everyone at the start of this course.
We use more exact prepositions when we don't want a clunky long s-genitive expression like den Europeiska unionens länder.
So a map of Sweden is en karta över Sverige, just to take one more example. Someone should make a list of all the various options, that would be fun. :D