Translation:If you say "firstly", then it is necessary to say "secondly" and "thirdly" later.
Firstly, this is the only sentence I've come across that requires non-letter symbols. Secondly, it's needlessly pedantic when it comes to perfectly serviceable alternative translations. Notice how I don't add a third point here, as an informal protest to this specific abomination of an exercise.
This question is a total nightmare because it requires a particular form of punctuation. I have found that when I type double quotation marks as required by the answer I am marked wrong. It may be that my BrE keyboard differs from an AmE one, because when I use single quotation marks I am marked as correct.
In the multiple-choice exercises, sometimes hyphenated words are in one choice bubble and other times they are in separate choice bubbles, and it is beyond infuriating. It's almost as if they want me lose hearts as a punishment for using the free version! The words i chose were totally correct.
I am not a native english speaker and probably not perfect. But I use English language now during more than 45 years and I cannot remember to have heard the expressions "firstly", "secondly" and so on. What I know is "First", "second", etc. Is here some - preferable - native english speaker, who can help me out?
Firstly, secondly, and thirdly aren't incorrect, they're just a bit stuffy. They're adverbs (note the -ly ending) and so in many cases where they might appear they are, in fact, incorrect. Technically. But you'd be understood, and most people would be afraid to correct it because it sounds so formal. In the US, we're intimidated by formal-sounding usage. I'm a little surprised that you've made it 45 years without encountering firstly, et al, but you haven't missed anything important.
There are also some phrasal variations that occur a lot. "First of all" or "First off" are equivalent. But in modern usage, things are typically streamlined to just "First."
Parallel construction is not a requirement. That is, the exercise isn't really applicable in English. People say things like, "First of all, ... and second, .... thirdly, ..." and it's fine. (In business or technical writing, parallelism would be more important.)
I imagine someone counting on their fingers,saying "firstly" with one finger extended, "secondly" with two fingers extended together, etc. You are probably better off never using them; modern style is to prefer shorter words and phrases, and "first" is shorter than "firstly".
I think this garbage question is the final straw that makes me abandon Duolingo. This Russian course is broken. There are so many bad translations, so many reports of things to be fixed but it seems even the admins have abandoned ship. I've liked this app for quick practice sessions but enough is enough. Into the bin it goes.
No one says "it is necessary," in colloquial American-English (I also don't believe in any English dialects). Especially because the subject has already been established as "you," we would say "you need to." Even without that however, we would still most likely say "you" in the rhetorical sense. If you wanted to get really fancy, you could say, "one has to/must," but "it is necessary" sounds too bookish.