Why is it なさそう and not なそう


"It seems there is none" is なさそう, which escapes the usual rule for 形容詞
(イ-adjectives), which says "drop the い and add そう".

Is there a historical explanation for this exception? And does it have
anything to do with なさいません?




Your "usual rule" is incomplete. It should be:

  1. drop -i
  2. if resulting is a single mora in length, add -sa
  3. add -sou.

Hence, nai:

  1. na
  2. na + sa
  3. na + sa + sou --> nasasou.

atui:

  1. atu
  2. (not applicable)
  3. atu + sou --> atusou.




Your "usual rule" is incomplete. It should be:

  1. drop -i
  2. if resulting is a single mora in length, add -sa
  3. add -sou.

Hence, nai:

  1. na
  2. na + sa
  3. na + sa + sou --> nasasou.

atui:

  1. atu
  2. (not applicable)
  3. atu + sou --> atusou.

