I see that you are learning Dutch,so it might be interesting for you to know that we can also say "dat is in overeenSTEMing met de realiteit" which means 'that is in accordance with reality",which also means 'das stimmt'
Furthermore the dutch word for voice is 'stem' like the german 'Stimme'.
I believe "Det är så" is the Swedish equivalent of "It is so" and Duo wants to keep them as separate expressions.
But while we're at it... When you say "It is correct", it means "it" agrees with a certain norm or convention. Whereas, when you say "It is so", it means "it" corresponds to the way things are (reality). Technically, "it is correct" is a judgement and "it is so" is an confirmation of reality/truth. But since, in practice, reality and truth are ruled by perception, it is all judgement. head explodes
Do you mean the noun? It is somewhat related.
The verb "att stämma" have three meanings (unless I forgot something):
1.Beloppet stämmer - the amount is correct
2.Jag stämmer företaget - I sue the company
3.Jag stämmer pianot - I tune the piano
"En stämma" means a voice, usually as in e.g. soprano, so it is related to 3.
I cant really think of a good noun for 1, but for 2 a lawsuit is "en stämning" (which also means ~ambiance, it shouln't be too easy :-))
I find it easier to think of this verb as "stand", as in, "that will stand" or "that will not stand" (in court). It probably would be a horrible way to translate it, but it helps me remember it.
But if I wanted to say "I will not stand for this!", what would be the best way to say it?