I commonly mess up these two words and other similar because I don’t see the difference between the long おう (as in おとうさん、おとうと）、ice (氷 - こおり、かきごおり）。 I can’t really think of any more examples with long おお, I believe 大いに and such don’t count.
Does anyone know why does this change, instead of 氷 being: こうり？ It seems like it would have to be also correct?
Thanks a lot, this is my first post I hope it makes sense to people.
Long and short vowels are important in Japanese, probably more important than in your native language. You have to learn it explicitly in the beginning, there is no specific reason why there is a short or long vowel. Just listen closely to the audio and after a while you will start hearing the difference. People will be really confused if you don’t follow it while speaking because many words already sound similar to begin with, without making mistakes.
The sound changes are also important, you can’t just choose what to do because you might land at a word with different meaning.
You’re right, I can’t seem to find it exactly mentioned. Basically the part that mentions it is: "By Late Middle Japanese, when these sounds were inside of a word, /ɸ/ became [w]. " And then these [w]s eventually fell off in the same way as ゐ and ゑ.