so + adjective implies a general exagerated quality (abstract) that + adjective implies a comparison to a specific quality present (concrete) Example: You are so bad (you are very bad) You are that bad (you are as bad as (context)) Why so fast? (Why very fast?) Why that fast? (Why as fast as (context)?) "that + adjective" I will give further examples The car was going 75mph, why that fast? "why that fast"- why as fast as 75mph?" he uses a limo, yes he is that rich.

Isaac’s answer to a good explanation of the that/so topic. To answer your basic question, there is no “rule” or common usage that distinguishes “so” from “that.” The distinction is only the one Isaac mentions, about how specific the comparison is. One last note. “You’re not so clever.” would often be said, “you’re not as clever as you think you are.”

Thank you.

In this instance, neither of the options sounds natural. Maybe if the example was "You're not so (that) great." Both imply that the person isn't as wonderful as they are pretending to be. I would personally use "You aren't that great" or "not so smart". It allows for a scale, where "right" is either true or false.