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

Question

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.

Answer 1

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:

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.