～そう

〜～そう on the ます-stem of a verb or the stem of an adjective says how something looks to you: "looks", "looks like it will". It is your impression from the outside, of something present or about to happen. An い-adjective drops its い; a な-adjective is used bare.

～そう behaves like a な-adjective: そうだ or そうです at the end of a sentence, そうな before a noun, そうに before a verb.

Not the "I hear" そう

After the plain form, そうだ means "I hear". The attachment form decides which one you have.

• × 雨が降るそうです (for "it looks like rain") → 雨が降りそうです

Only for guesses

A thing you can see for certain is stated directly, and a noun never takes this ～そう.

• × この花は赤そうです → この花は赤いです
• × 彼は学生そうです → 彼は学生のようです

The impression is about now or what comes next. For something that looks as though it has already happened, use ようだ.

いい, ない and negatives

いい and ない take さ: よさそう, なさそう. That covers adjective negatives too, which end in ない: 〜くなさそう. A verb's negative is そうにない or そうもない.