Why are there so many kanji for こう? I’m on level 5 only, and I must have seen close to 15 kanji with this reading, plus a couple with variations on it (e.g. ごう), in just the earliest levels! I’ve been listening to Japanese for ages and I don’t recall there being such an extensive use of this particular grapheme in the spoken language.
…is this Koichi’s way of slowly brainwashing us…? O.o
It’s worth remembering that Japanese borrowed them from Chinese, which had tones, and so those characters that all got imported as こう were not identical originally. They only became indistinguishable when you crowbar them into a language that doesn’t have tones. So… good job ancient Japanese monks, no individual of which could have possibly foreseen how their interactions with China would confuse foreign language learners hundreds of years later.
It gets worse. 
Other than three Kanji that all had すい readings, my last review was mostly こう for 皇 (except for 天皇 in which it becomes のう).
Good times. lol
I was about to post the same; at level 5, and so far it seems half the kanjis are read either as ko-u or hi. This is good for now, as in most cases I don’t remember a reading (for kanji, not vocab), I can cheat - it’s likely one of those.
But I don’t know how long-term this will be, especially as spoken Japanese doesn’t seem to have that many of these…
What do you mean by that?
I think you mean こういう意味ですか 
I mean the proportion of ko-u and hi readings seem much higher than what I hear in spoken language, just as the original poster said.
The explanation of why there are many ko-u readings makes sense, but I wonder what its effect is (mostly on learning japanese). E.g. how easy is to differentiate the reading into the appropriate kanji/vocab.
Hmm. Words with こう in them aren’t particularly rare or anything.
For instance, がっこう, りょこう, くうこう, and こうえん are words that most beginners know (and are used plenty in spoken language), and all of them use a different kanji for こう (校, 行, 港, and 公)
In written language homophones can much easier be distiguished from eachother through use of kanji. Spoken Japanese tends to use more kun’yomi, rather than on’yomi words than written language does. That said, こう is still common to hear.
I never thought ひ was a particularly common sound, though. Apart from 日 and 火, both kun’yomi, I can’t think of any other words you’d have encountered at level 5…
Oh, they knew, they did it on purpose! On purpose, I tell you!
Yes, he is! But don`t worry - once you are nice and brainwashed into crabigatorism you will be allowed to learn more of other kanji with non-こう readings.
Hmm. Words with こう in them aren’t particularly rare or anything.
Rare, no, of course, I didn’t mean that. But they don’t seem to me as common as the amount of kanjis at these low levels that have such a reading.
Another one that starts coming up really frequently is き. Also しょう and せい.
It kind of makes it easier too though, especially when working with mnemonics. For some kanji I just go “Ah yes, fire-axe-surround, no idea what my story for that was, but it somehow makes me think of the shogun, so しょう”. (that’s not a real kanji btw, just a demonstration )
EDIT: Whoops, I did not mean to reply to you, but just to the thread in general, sorry!
Honestly, that’s better than the other way around.
人 can be read as にん, じん or り, depending on the vocab.
日 becomes ひ, に, か, にち or じつ for the same reason.
Not to mention that I don’t wanna count how many times I’ve seen items get downgraded because of rendaku or because I forgot a lengthening vowel.
I never thought ひ was a particularly common sound, though. Apart from 日 and 火, both kun’yomi, I can’t think of any other words you’d have encountered at level 5…
There’s also 皮 (L5, On’yomi reading), and 引 (L3, Kun’yomi reading). I might miss one or two, since it’s not easy to search for readings.
Maybe it’s not a lot, but it did surprise me as well.
or because I forgot a lengthening vowel
Right, me as well 
こう and しょう are the two most common on’yomi across all kanji.
I checked my kanji dictionary - of the 2882 kanji listed, 111 have the reading こう, or a bit under 4%. And granted, 4 of the 43 in kanji in level 5 have that reading, which is closer to 8%, but across all the levels 1-5, it’s 7 out of 170, a bit over 4%, so in actual fact, you’re coming across こう here only slightly more frequently than you would in the real world. And level 6 has no こう, dropping the overall percentage to less than the average.
Well, to be fair you did say it “seems” like “half” of the kanji are kou or hi… So, your percentages are a little off
Anyway, it just sounded like you were suggesting those kinds of words I mentioned weren’t heard much in spoken language or something. There are plenty more than that as well, it was just a few examples.
haha I was just about make a thread about this. When I can’t remember the ON reading during a review, I always guess kou…
It’s scary how often that’s right.
コウイチ is everywhere you most or least expect him… It’s the places in between where you have to worry.
This doesn’t make sense, so don’t try to find it!