It's all about context. And yeah; English can be confusing.

"Hot" is good. But so is "cool."
If you're interested in a particular activity you can either be "up for it" or "down with it."
Inert” means (among other things) lacking any chemical reactions. However, there is no word “ert” to indicate the opposite.
“Inhibit” means to prevent or discourage from doing something. There is no word “hibit”.
If something is “inverted”, it’s upside down. However, if something is right-side up, it’s not called “verted”.
I’ve met a number of disgruntled people, but I’ve never met someone that I could call “gruntled”.
I’ve heard awkward people described as “ungainly” or “inept”, but I’ve never heard someone described as “gainly” or “ept”.
Then there are the words “flammable” and “inflammable”. You’d expect that adding in- to “flammable” would produce its opposite. In this case, they mean exactly the same thing!
Depending on the dictionary and how thorough it is, the word "run" can have over one-hundred and fifty definitions.
English is the Heinz57 of languages in that so many words have been borrowed or Anglicized.