How to know which On reading to use?


Just something as simple as the character for "one" has the On readings "ichi"
and "itsu." So how would I know which one to use if I saw this as part of a
compound?




Unless someone knows something that I don't, you just have to learn it for
each word. Sometimes a word's pronunciation has evolved so that 9 times out of
10 it's pronounced one way, and that 10th time its the other. For example,
it's a pretty safe bet that the On-yomi for `一` in a compound is `いち` or `いっ`
(depending on the sound following it), but every now and then you'll find it
read `いつ` (as in `単{たん}一{いつ}` which is so hard for me to remember).




Unless someone knows something that I don't, you just have to learn it for
each word. Sometimes a word's pronunciation has evolved so that 9 times out of
10 it's pronounced one way, and that 10th time its the other. For example,
it's a pretty safe bet that the On-yomi for `一` in a compound is `いち` or `いっ`
(depending on the sound following it), but every now and then you'll find it
read `いつ` (as in `単{たん}一{いつ}` which is so hard for me to remember).

