In all of the above words, it appears that the 未然形 has the
-a- surface form.
However, in these three words, it seems that it has the
-o- surface form:
What I'd read previously is that
-o- are both considered the same underlying form because of the sound change /au/ → /o:/. In other words, the
-o- surface form is underlyingly
-a-, but changes to
-o- as part of /o:/.
However, these three adjectives do not contain the long vowel /o:/, so I don't think that explains why they have an
-o-. And if this is the result of a sound change, it doesn't appear to be a regular sound change, because most adjectives have
-a-. In fact, both
So how can these
-o- forms be explained?