"Personne ne comprend pourquoi."
Translation:Nobody understands why.
I agree. Sure, knowing and understanding are different, but to my knowledge no one really says "no one understands why" in English anymore. For this phrase, at least, it seems that we've pared it down to using "no one knows why", regardless of whether or not our word choice is completely accurate.
There are double negatives in French as well, but they are intended:
- je ne veux pas ne pas parler (je ne veux pas me taire) = I don't want to keep quiet
But basically, French negative constructions have two words:
- not = ne... pas
- not any longer / no longer / not any more / no more = ne... plus
- never / not ever = ne... jamais
- nothing / not anything = ne... rien
- nobody / not anybody = ne... personne / personne ne
In French, negatives are formed in pairs. It takes some memorization, but usually it's ne+some other word. 'Ne [verb] pas' is the standard don't, but there are also ones like 'ne [verb] plus' which means no more, 'ne [verb] jamais' which means never, etc. For this sentence, ne is paired with personnne. 'Personne ne [verb]' means nobody.
"No one" and "nobody" are accepted equally. "No-one" as a hyphenated form does not exist. http://dictionary.cambridge.org/us/dictionary/english/no-one