So, there’s this sentence:
15時間も寝たので、頭が痛くなった。
With translation:
I slept for 15 hours so I have a headache.
However I can’t figure out what the hell does も do there? It can’t be “also”… it would make no sense…
Also, as a side question, why is it ので, rather than から ? Kara seems more suitable there. Or am I wrong?
Another use for “mo” is to mean “as much as”. I don’t have IME on this computer so romaji will have to do. See this example sentence.
okaasan ha neko wo sanbiki mo katteimasu
It means “Mother keeps as many as three cats”. It is often taught together with “shika~~nai” which means as little as.
So, that’s what it is! Thank you for the explanation. I will go refresh my memory about the “shika” point and also learn this additional “mo” usage.
Though, could you possibly also comment on my another point in the original message?
As Kumirei pointed out, も can be used to mean “as much as”. Here’s a brief explanation from Genki: http://wp.stolaf.edu/japanese/ressource-projects/genki-i-ii-grammar-index/number-mo-number-shika-negative-genki-ii-chapter-14/
Not sure why they’d use “node”. As you say both version work and from what I’ve learned “kara” is more casual and would indeed fit better. I assume that it’s just how they chose to word it and not that “node” is the correct word to use here, but I really don’t know.
I just found this:
Like から (kara), ので (node) indicates cause or reason. While から (kara) subjectively presents a cause or a reason, ので (node) highlights a cause-and-effect relationship as a natural course of events.
DiMono, thank you for the link.
Hmm. Yeah, I can see that nuance now that you say it.
I found this small guide very helpful for particles (even though it’s slightly different from your question)
http://www.fluentu.com/japanese/blog/japanese-sentence-particles/
Other posters have done a good job of explaining this use of the も particle.
But the sentence you posted should be translated as:
Since I slept for 15 hours, I DIDN’T have a headache.
痛かった = was painful
痛くなった = wasn’t painful
I think you’re mixing up
痛くなかった (or 痛くなくなった)
and
痛くなった (using なる, to become)
The former means “wasn’t painful” (or “stopped hurting”)
The latter means “started hurting”
痛くなくならなかった = didn’t stop hurting
D’oh! Here I thought I was being smart for the first time on this forum. 
Thanks for the clarification.
We all make mistakes 
Ooh, how does one insert a bonus?
Also, thank you for your earlier (second/third post) second use for も!
[quote=“AnimeCanuck, post:13, topic:16466, full:true”]
Ooh, how does one insert a bonus?[/quote]
My pleasure 
Oh… is it the Hide Details? Thanks!
EDIT: Oh you said so… I have to get better at reading, apparently. XD
Made it more visible for you 
*shoves you away*
I saw it all already… XD
God I loved his character, so cute.