conjugations - What is the difference between [plain form of verb]～そ う and [root of verb]～そう?

I heard both forms of [plain form of verb]～そう and [root of verb]～そう in an anime I watched, reproduced below, so I'm wondering how are they different and how to choose to use one over the other?

行けるそう
行けそう

What are these conjugations called? And how do their negative forms look like?

Answer

そう after the plain form of a verb indicates you are reporting secondhand information, rather than your own direct observations. 行けるそう means "it is said that it can go." The negative form is -ないそう: 降らないそうです "They say it won't rain."

-そう after the stem (-ます form) of a verb means "looking/sounding as if ___." 行けそう means "it looks as if it can go." It is similar to constructions like 行けるみたい or 行けるよう, but -そう emphasizes evidence of the senses rather than general "seeming." The negative form is -なさそう: できなさそう "It looks as if s/he can't do it."

(Explanation paraphrased from Japanese: The Spoken Language)