Translation:She suddenly left, we don't know why.
why not 突然地?
I mean, what's the difference from 她高兴地笑了 or 她生气地离开了?(https://www.duolingo.com/skill/zs/Personality-and-Feelings/tips)
You use 地 to turn an adjective into an adverb. In the case of 突然, it is already an adverb so you don't need 地 when you use it before a verb.
I don't know of an easier way to figure out if something is an adjective and needs 地 or it is already an adverb by default. I think it's just something you have to learn as you go along.
Although google translate also shows parts of speech, I've found Yellowbridge is more informative and has less incomplete entries. That being said, some of their parts of speech entries are also incomplete.
For instance, the entry for 瘦 (shòu) says its only used as an adjective. However, I've also seen it used as a verb in some Duo sentences. When you look at the definition it says: "thin; to lose weight." The "to" let's you know that it's actually used as a verb in Chinese. It's the same with 认真 rènzhēn (serious).
Despite these small issues, it definitely helps a lot.
I really think 他突然离开了我们不知道为什么 should be accepted. There is no way we should be able to infer the gender here. I have had the same problem with other sentences, in which there were "hints" that suggested the gender, but those hints were straight sexist or homophobics. Only a woman can have a husband, flowers can only be gifted to men, etc