Wouldn't the use of passive be correct here: "some things cannot be explained", I believe it has the same meaning since "man" is a rather general or impersonal pronoun.
It would have almost the same meaning, but I think the course makers want you to translate every word, so you learn to use them. Your sentence would be better translated from "Noen ting kan ikke forklares". Which would have almost the same as the current Norwegian sentence.
Nice! even though the sentences were not exactly the same it made sense to me to accept is as a possible option
Think of "man" as "one". Without "man" in this sentence, the translation would be "Some things cannot explain".
The Norwegian word "ting" is written the same way whether it's singular or plural.
En ting = one thing
Flere ting = several things
So why is the relative clause word order used here. I always interpreted "man" in this sentence as the subject and "noen ting" as the DO.
I find it rather annoying that "Some things man cannot explain" is supposedly wrong even though in this context man and one are interchangeable.
As far as I can see the translations, you can translate the norwegian "man" as "you" or "one". If you use the english "man", it would have been "mann" in the norwegian sentence.
Either way, I think the english translation you suggested seems a bit off. If you meant "No man (in comparison to woman) can explain" or "No human can explain", I would write "Some things men cannot explain.". That would be "Noen ting menn kan ikke forklare.", I guess?! Please note the different meaning in comparison to the given sentence for this exercise.