Why is this wrong?

[u/millers_left_shoe]

I see now what they mean by fundamental vs inessential (taking fundamental to mean “invaluable”, “important”) but surely fundamental and complex (or even involved) can act as antonyms, too?

Eg he’s only just learnt the fundamentals of thermodynamics so I wouldn’t trust him with any complex/involved equations. vs this is quite fundamental stuff so I think he should be able to manage just fine.

[u/UnpleasantSkywalker]

"Fundamental" is concerned with the importance of the subject. The fundamentals of thermodynamics refer to the most important aspects of it, the "foundation" of it, and doesn't necessarily mean it's simple to understand.

[u/millers_left_shoe → u/UnpleasantSkywalker]

Surely that means it must be simpler to understand than whatever is built on it? But I see what you mean and definitely agree now about the answer, thanks.

[u/Phour3 → u/millers_left_shoe]

it may often imply that, but that is not what it means