What is the crowd's opinion of Oppia?

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> Learning your conlang for yourself.

This can be achieved simply by using it a lot. Doing lots of translations or journalling/taking notes in your conlang should help you become pretty efficient in it. Obviously, it's gonna take time and practice, but it'll come eventually.

> Teaching the conlang to other people.

First, you'll have to find people who want to learn the conlang, and that is unfortunately rather unlikely, especially if you're just starting out. Not many people want to spend all the time and effort to learn a conlang beside maybe being able to say a few simple phrases. This is because conlangs aren't nearly as pragmatic or useful as, say, a natural language. I'm not saying you can't; Toki Pona and Klingon, for example, have rather large fanbases, and Esperanto even has native speakers. However, several different factors contributed to their success.

As for lessons, some options are Memrise, Oppia, or to just write the lessons yourself and share them as PDFs or publish them on a personal wiki site like Miraheze. You could also start your own subreddit or host a discord server to communicate directly with learners.

For now, though, the best thing you should concern yourself with, in my opinion, is learning more about language and linguistics, making sure your conlang is ready to be shared, and then learning how to effectively present and teach your language.

Looks like there also a similar thing for predicate logic. I made it all the way to the final question before getting stuck (because I couldn't make sense of the error message).

Unfortunately, there is never any introduction of the rule for double-negation elimination in this or the one linked in the OP. While double-negation elimination is redundant here, it's pretty important to have so that you can avoid doing excluded middle every time you want to prove a non-negated statement by contradiction.

That being said, this was fun to play around with for a little while to waste some time.

I have created this exploration using Oppia. Initially, the exploration creation interface is bit confusing. Anyway, it won't be an issue if we are familiar with it.