Why does the い-adjective require -さ conversion to match ～そうです？


I'm doing an example JLPT test, and I came across a sentence which required a
blank to fill in. The correct version (according to the answer sheet) of the
sentence is:

少し事情を説明したほうが **よさそう** だ。

Now, another option, which seemed correct to me was:

少し事情を説明したほうが **よそう** だ。

Why is the 2nd form incorrect? I always thought I can just drop the い and add
そうです to the stem. Why do I have to transform the adjective into noun here?
Thanks for clearing that for me.




You are right; い-adjectives (except for negative -ない endings) do _not_ take a
さ before そう.

いい is actually just irregular; it becomes よさそう. That's just the way it is.

Here's a complete list of conjugation rules for the -そう grammar pattern,
quoted from [Tae Kim's Guide to
Japanese](http://www.guidetojapanese.org/learn/grammar/similarity):

>   1. Verbs must be changed to stem.
>   2. The 「い」in i-adjectives must be dropped except for 「いい」.
>   3. 「いい」must first be conjugated to「よさ」.
>   4. For all negatives, the 「い」must be replaced with 「さ」.
>   5. This grammar does not work with plain nouns.
>




You are right; い-adjectives (except for negative -ない endings) do _not_ take a
さ before そう.

いい is actually just irregular; it becomes よさそう. That's just the way it is.

Here's a complete list of conjugation rules for the -そう grammar pattern,
quoted from [Tae Kim's Guide to
Japanese](http://www.guidetojapanese.org/learn/grammar/similarity):

>   1. Verbs must be changed to stem.
>   2. The 「い」in i-adjectives must be dropped except for 「いい」.
>   3. 「いい」must first be conjugated to「よさ」.
>   4. For all negatives, the 「い」must be replaced with 「さ」.
>   5. This grammar does not work with plain nouns.
>

