How do you know this?? is there a rule, or do you always have to look up in the dictionary??
Sorry if my questions are silly, but I want to improve my understanding of the language more and more… unfortunately I don’t know any Japanese people who can teach me, and all I have is this forum… and I’m still just level 9, so my knowledge of Japanese is very limited for now 
Basically you have to learn it on a case-by-case basis.
It’s just a quirk of how the Chinese writing system was mapped onto a preexisting language that had very little in common with Chinese.
Sometimes the same Chinese character would fit multiple Japanese words, like 空 above. Or 角 fitting both かど (corner) and つの (horn).
Sometimes the opposite would happen and the same Japanese word would fit multiple Chinese characters, in which case these take on different nuances, like あう being written as 合う (match, suit) or 会う、逢う、會う (all being meet)
Or いたむ being written as 痛む、傷む (suffer physical damage/pain) or 悼む (suffer emotional pain, i.e. grief)
So for me, yes, I should still look up in the dictionary, in order to know which reading to use for the same kanji in different contexts… but you, as a level 60 WK student, do you need to??
Do it if you want to 
Personally, I’ve pretty much accepted long ago that when I read there will be a lot of words I’m not sure how to pronounce and it bothered me at first, but now I’m fine with it.
Others might prefer to know for sure and look up every unfamiliar word.
My advice would be to at least keep in the back of your mind that when learning a language, getting the gist is actually enough! If you want to know more, then go for it and look it up, if you feel that you’re enjoying what you’re reading just fine without knowing exactly how it’s pronounced, or it’s exact meaning, then that’s okay as well. It may vary from moment to moment as well.
It even happens to Japanese people. I asked a Japanese friend how a (perhaps made-up, I couldn’t find it in a dictionary) kanji compound word in a book should be read and she didn’t know. Same with names that don’t have Furigana. She reads a lot but seems to just roll with it
Again, it’s a quirk of the way Japanese uses essentially foreign symbols for writing a pre-existing language that you can never be sure what the reading is when you see a new word.
Maybe it’s kun when you think it’s on? Maybe it has a unique reading in this instance? Maybe it can have multiple valid readings, like 家 / 家 or 剣 / 剣…
I love this post!! Interesting how even Japanese people don’t know how to read their own language 
To be fair, you’ll sometimes hear native English speakers unsure how to pronounce less common English words as well.
It might just be easy to forget that English has a lot of quirks and irregularities as well that we take more or less for granted. (Record is intonated differently when it’s a noun and when it’s a verb, Kernel and Colonel are pronounced the same, Word, Sword and Sworn show how the same sequence of letters can be read quite differently depending on what comes before or after…)
And a lot of people had no idea how to pronounce Hermione when the Harry Potter movies hadn’t yet been made!
Probably a lot more common with Japanese though.
What happens in Japanese is the same that happens in English.
Let’s say that English decided to use sinologograms, for some reason.
Then we’d have
食 - eat
豚 - pig
食豚 - eat pork
心 - heart
医 - medic
心医 - cardiologist
手 - hand
歪 - oval
手歪 - American football
Most major languages use another language (a “prestige language”) to form words. Japanese uses Chinese. Western languages use Latin and Greek. That happens for a variety of reasons. In the case of Japanese, they liked Chinese culture. In the case of English, England had French-speaking elites for generations (and they ate “pork”, not “pig”), and, in general, Latin was the academic language for a long time.
As an aside, there is actually kanji for American football. 鎧球 (がいきゅう, armor ball)
That was the initial intention. But the Royal Society of Logograms just couldn’t resist to piss the Americans off.
The Tofugu article is useful but simplified. Here’s more information.
The widespread use of Chinese characters to write Japanese predated the widespread use of (the Japanese pronunciation of) Chinese vocabulary within Japanese.
In the earliest examples of written Japanese, Chinese characters are strung together to form phonetic representations of native Japanese vocabulary. This system is now called man’yōgana. The Wikipedia page has concrete examples.
In the man’yōgana system, which we know only from a small corpus of texts, each distinct Japanese sound was assigned one main corresponding Chinese character. (Interestingly, the number of distinct Chinese characters used as man’yōgana seems to indicate that Old Japanese had more distinct sounds than later stages of the language.) Those characters, written in cursive, simplified form, eventually became the kana system.
Another nuance: The categories of “Chinese” and “Japanese” within the Japanese language have not been stable over time. Quoting from Sayonara Amerika, Sayonara Nippon, a good book about the development of J-Pop:
Naoki Sakai [a Japanese researcher] has argued that in the eighteenth century Japanese intellectuals reached a previously unknown epistemological relation to the language they used: they discovered that they were using a hybrid language, one composed of both Chinese and Japanese elements. […] The discovery of a stable image of an other—in this case, China—enabled the simultaneous discovery of an image of the self, Japan.
In other words, the neat separation of “Chinese words” and “Japanese words” was undertaken later than you might expect—the eighteenth century—as a step in the formation of a Japanese national identity.
Even though they use the same symbol it’s just that から is a Japanese word meaning empty and そら means sky. You’ll get used to it as you learn.
Of course when you’re actually using the language you’ll know which meaning is needed from the context of the sentence.
In spoken language you’ll easily tell and in written language you’ll just have to look at 空 and decide whether empty or sky makes more sense in that sentence.
In reverse it’s like how in English you can hear their there and they’re and know which the speaker means.
It gets worse. You will have native speakers straight up not being able to read store signage cause handwritten kanji is…a mess. It took me some years to realize that I was being too hard on my mom when I was trying to get directions and I asked her to read off a restaurant name and she straight up couldn’t. Like, it’s your language, why CAN’T you read it? Oh, it’s like cursive but sloppier? Oops.
So you could say that on’yomi are basically root words from Latin, Greek etc? For example ambi, in ambidexterity.
Yes, it’s like that sometimes and not other times.
Comparing on’yomi to root words in European languages is only useful when you’re saying why on’yomi can’t be used alone.
But the truth is that certain on’yomi can be used on their own.
You’ll break your brain if you try to understand all the whys about any language and sometimes it’s best to try to accept that it is how it is.
Learn kanji so you can learn vocabulary. Don’t learn kanji and think you’re learning vocabulary.
If anything is like root words it’s the kanji themselves rather than their readings.
空 as a kanji means nothing. It isn’t vocabulary. It’s essentially a letter (if you want to use familiar terms). A doesn’t mean anything until you surround it with FTHER. Similarly, 空 doesn’t mean anything until you give it a reading.
I don’t know that I’d go that far. Kanji can be used to represent concepts even when they aren’t meant to be read at all. If you see 引 on a door, it doesn’t matter whether it’s いん or ひ or whatever, it just means the door needs to be pulled.
You’re right! I didn’t think about that. I guess in that way it’s like using any other symbol 
though. 
Maybe it’s better to say that it doesn’t mean something until it fits into a given context. That context could be a word, or an associated reading, or like on the door for 引 implying pull, or (just making something up here off the top of my head) as an abbreviation for 引退 on a list of members of a club. As long as there is a context, it can be understood to have meaning, but if there is no context, it’s impossible to say precisely what it means.
I like that way of thinking about it. Once again you’ve managed to say what I wanted to say.
I think that because kanji is so different to what most of us were used to we get bogged down with it. Knowing a thousand or so kanji is useless without vocab (unless you only need to know what doors say!)
Maybe they were trying to make a comment on the meaningless of existence…nah, hahah.
I was in a stationery shop, looking for blank postcards to decorate and I think I asked for “air cards” when I was trying to say “empty cards”. I settled on 私のデザインの葉書. It did the trick.
That’s why I love kanji: you see it, you understand it, and you’re 大丈夫 