When to use 'on' and 'kun' readings when writing kanji?

Question

In a Japanese sentence they often use kanji, but how do they know when to pronounce these kanji with their 'on' or 'kun' readings?

e.g. 友達 = tomodachi, 友 = tomo, 達 = dachi

but 友 also is 'yu' which is the On reading, and 達 = tatsu which is th Kun reading. You can see when put together they mix the On and Kun. Is there a formula that automatically tells you when to use a certain reading, or do you have to learn these kanji phrases by heart as you go through schooling?

Thanks! =)

Answer 1

reading is depends on the combinations of each characters.

so you need to memorize lots of idioms to read characters.

basic rule when you read idiom

1. idiom with on + on reading

2. idiom with kun + kun reading

on + kun or kun + on combinations are rare.

Answer 2

When you enter school as a Japanese kiddy, you already speak the language. Meaning, you already know it's "tomodachi" and not "yuudachi"

So, they just learn the on/kunyomi.  

I just memorized all the kun/onyomi, there was no special trick that Kimiko taught me. However, Kimiko was a gift from God because she made me by a kanji book with all the kun/onyomi in it for the 2000 most common used kanji there are. God bless her soul.

"A New Dictionary for Kanji Usage" if you're interested. If not, I can't advise a better way for you to memorize the readings.

Sorry.