Translation:She suddenly left, we don't know why.
She left suddenly was counted wrong. It sounds better with the adverb after the verb.
I think they emphasize different things. One highlights her 'goneness' while the other highlights the 'suddenness'.
This is correct English, but because the Chinese sentence is separated into two different ideas, the given translation is better. 我们不知道为什么她突然离开了 would bs a better Chinese translation for your sentence. =-]
Honestly I'm pretty annoyed at this right now. I was given this as a "type what you hear" question, without a word bank. Got it wrong for using the male 他 instead, even though there is no way to know if I'm just hearing it.
Because of Chinese grammar in time sentences this can also be a past sentence you could say don't know why but realistically it would actually be did not know why
No, that's for the "leave (left)". They still don't know why, otherwise there'd be more indicators e.g. 我们那时/当时（还）不知道为什么。