In the sentence "Rien ne peut arrêter ça." [Nothing can stop it.] the word "ne" seems to me to be doing nothing. It seems like the sentence should be "Rien peut arrêter ça."
Why doesn't "ne peut" mean "can't stop"? Why doesn't the initial sentence mean "Nothing can't stop it."? Or even "Nothing won't stop it."?
Is it because "ne" doesn't negate words with an already-negative connotation?
I am pretty new to French, sorry if this is a stupid question.
You can hear rien peut arrêter ça in spoken French with the expected meaning.
In written/formal French, a negation is built with a split negative form composed with the adverb ne and a pronoun like rien, pas, personne and the likes.
These pronouns come from nouns with a (possibly lost) positive value (rien=something, pas=step, personne=person) so this isn't a case of double negation.
Etymologically, rien ne peut arrêter ça, translates to "(a) thing can not stop that", i.e. "no thing can stop that".
Note that prepended with an article or a few other cases, these pronouns become nouns with no negative value :
No, ne does not create a double negative in French, no matter what is paired with it.
Rather, the pair ne ... pas is only one negative. You can substitute several other items for the pas, and you'll get a different "flavour" of negation with a different meaning.
This answer contains a partial list of negatives, including ne ... rien.
Note that ne is often dropped in spoken French, which might give the impression that it's contributing an extra negation when present. This is not the case.
If you wanted to double-negate rien, you would have to add the element that is actually contributing negation: pas.
Rien n'existe pas. [Nothing doesn't exist.]
