I just finished RTK1, and I'm still deciding between reading RTK2 or RTK3.

I have a simple question for RTK2: in the first chapter (the kana and their kanji) the 2nd kanji (三) has a kun-yomi (I checked that with jisho.org) even though it's in the part called "Chinese Readings." Is this done on purpose because it's a kanji that's the basic part of a kana?

Exactly. Some kana reading come from 音, some from 訓, and some come from readings that aren't used anymore outside of names.

Thank you. I have a last question about on/kun readings (I promise).

If I understand correctly, "on" readings are Chinese pronunciations mostly used in compounds, "kun" readings are Japanese pronunciations mostly used in verbs and adjectives, and (here's the thing I need an explanation if I'm wrong) :

on/kun readings have absolutely no grammatical "responsibility", this is just about the way to read a kanji.

Am I right?

mypapa12 Wrote:Am I right?

More or less. It's not a hard rule. 水道の水 means "tap water," like you'd expect, and the compound is 音 and the single-character word is 訓, like you'd expect.

But, different readings may have different meanings. This is almost the rule for characters with multiple 訓, and not unheard of for 音 either.

楽, for example (examples from dictionary and may not sound natural)
ガク = music: 音楽 (music) 楽器 (musical instrument)
ラク = relaxed: 安楽椅子 (easy chair)
たの.し = enjoy: 楽しい (fun) 楽しみに待つ (look forward to)

If you don't know, guess 音--the less common the word, the better this works. You should still look it up if you can do so easily.

Otherwise you might be tripped up by something like 十人十色. Looks 音, but it's actually half-and-half.

Compounds "generally" use On readings but it's not a rule as such because compounds can be in any of 4 configurations 音と音　音と訓　訓と訓　訓と音。 I actually advocate learning to read via actually trying to read stuff as it will give you a very natural feel for which is the correct way to read words, something which RTK2 will not. IMO it's an important skill especially when you want to make a quick dictionary look up and need to guess how the word is read.

You don't really have to learn how to tell them apart, this is picked up automatically in no time. Generally, an on'yomi reading is just one or two mora long, while a kun'yomi is more.

Examples of common on'yomi: chou, sei, shou, ka, gen, hou
Examples of kun'yomi: tadashii, ugoku, musubu

So just from seeing the kanji and hearing how the word is read, you can usually easily tell if it's on or kun. If there's hiragana after the kanji, it's in like... 95% of all cases kun'yomi. There are however loads of exceptions of course.