Same here. That probably shouldn't be accepted. Unfortunately, the report button has no "My sentence shouldn't be accepted." option.
for the English speaker muss nicht and darf nicht seem to be swapped over must not = darf nicht, but if you remember that "must" is part of the verb with the infinitive "to have to" then it makes sense
I wrote 'you mustn't go right away' and it was accepted as correct, but the DL answer doesn't mean the same at all. So this leaves me a little confused, but further research suggests that I was wrong, and that the sentence denies that there is any obligation to leave, rather than saying that there is an obligation not to leave. (Though apparently some native speakers are sloppy about this.)
As far as I know "must" ~ "have to" and "must not" ~ "may not". German "müssen" ~ "must" but "nicht müssen" ~ "not have to" and "nicht dürfen" ~ "must not".
"they" should be okay. Maybe they didn't like the "go"? In such a context "gehen" means "weggehen" (= leave).
Definitely not standard usage lol. It's said, but they can't predict 100% of the answers people would give. Just stick to the more standard textbook usage and you should be fine.
They have not to go right away. In the sentence Sie refers as They since the verb müss is in plural.
Here "Sie müssen" can be "They have to" AND "You (formal!) have to" as well.
It can't be "She has to", because that'd be "Sie muss".
Without context or "Sie" being not the first word of the sentence you can't say if it is meant to be "they" or (formal) "you".
Only sentences like "Ich denke, dass sie nicht sofort gehen müssen." or "Ich denke, dass Sie nicht sofort gehen müssen." are unambigous.