43555
I am learning hiragana and I've encountered a minor problem. The thing is that
my book did mention how to deal with a long "O": just by adding another "O" or
"U". However it doesn't explain in which cases I should use which.

As you can see I've made a mistake trying to put "O" in 7th case. In which
cases of double "O" should I use "O" or "U"?

[![My
problem](https://i.stack.imgur.com/hF242.jpg)](https://i.stack.imgur.com/hF242.jpg)
CC BY-SA 3.0
2017-02-15T17:10:35.317
43552
2017-03-22T22:57:27.727
2017-02-15T22:55:56.093
1628
18520
question
hiragana spelling rōmaji long-vowels
Long O — when is it OU and when OO?
Really, all I can say is 'it depends on the word'. Generally on'yomi (Chinese-
derived) readings use おう, while kun'yomi (native Japanese) readings use おお,
but there may be exceptions. A note: if う is a verb ending, おう will not be
pronounced おお but as お and う separately, as in 追う and 思う. A lot of what I've
said also applies to えい and ええ.
CC BY-SA 3.0
2017-02-15T17:44:26.593
43555
2017-03-22T22:57:27.727
2017-03-22T22:57:27.727
9971
9971
43552
answer
Most of the time it is おう In rare cases, it is おお examples 遠く、通る おお also used
for the kanji 大 or 多 , so 大きい、多い
CC BY-SA 3.0
2017-02-15T18:16:10.250
43557
2017-02-15T18:16:10.250
19878
43552
answer
As @Nothing at all notes, this depends on the word.

_On'yomi_ always use _-OU_ for long O sound and _kun'yomi_ almost always
_-OO_.1

However, the real problem here is that you are being asked to reconstruct
_hiragana_ from Hepburn romanization. In general this is impossible, because
Hepburn romanization conflates certain _hiragana_ spellings. (There are
romanization systems that don't.)

The fact that there are many homophones is probably one of the reasons that
_kanji_ still exist in Japanese. Without _kanji_ and only _hiragana_ , we
would have that 糖衣、東夷、当為、等位、… would all be とうい.

Hepburn romanization goes even further, and also conflates とうい (e.g.
糖衣、東夷、当為、等位、…) and とおい (e.g. 遠い) to _tōi_.

So, you would need a lot of information to know that there is no word しょおゆ, so
that _shōyu_ would have to be しょうゆ. However, there would be no way for you to
know (without context) whether _tōi_ should be とうい or とおい.

The only way to solve this type of question is to know the _hiragana_ spelling
of the words that come up. (Well, it suffices to know the words in a
romanization system that does not conflate _hiragana_ spellings.)

All of that said, a long O (in Hepburn _-ō_ ) is more likely to be _-OU_ , so
the most economical approach would be to default to _-OU_ for _-ō_ and learn
words with _-OO_ such as とおい、とおる、おおい、おおきい、… as exceptions.

* * *

1 For _kun'yomi_ there are rare exceptions that arise from a sound shift as in
妹【いもうと】 from いも+ひと which also occurs as a long O sound in 弟【おとうと】, 素人【しろうと】,
客人【まろうど】, 若人【わこうど】, 蔵人【くろうど】. Across "word boundaries" Hepburn romanization
also uses _-ou-_ ; the above exceptions used to be word boundaries, but now
they are not anymore, so they have a long O sound, romanized as _-ō-_ in
Hepburn.)
CC BY-SA 3.0
2017-02-15T21:10:59.783
43564
2017-03-22T20:02:50.980
2017-03-22T20:02:50.980
1628
1628
43552
answer