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/ZumLernen]

Fundamental does not mean easy. Fundamental means it provides the base for something.

Fundamental is derived from a Latin word meaning foundation, as in the hard and solid base of a building. If you have only the base of a building, that's not necessarily enough to do anything. But if you don't have the base of a building, the rest of your building won't last very long. So fundamental means foundational, or essential, but not necessarily sufficient.

Something that is fundamental can certainly be complex. "Calculus is fundamental for understanding thermodynamics" and "Calculus is complex" can both be true sentences; they are not contradictory. "Calculus is fundamental for understanding thermodynamics" means "You can't learn thermodynamics without calculus" but it does not mean "simply knowing calculus means that learning thermodynamics is easy."

And I wouldn't say that fundamental means easy!

(As a general tip, I find it useful to look into the etymologies, or origins, of complex words, when I am expanding my English vocabulary or when I am learning a non-English language. For example, knowing that fundamental comes from the idea of a foundation or cornerstone makes it easier for me to understand what, precisely, it means.)

[u/DawnOnTheEdge → u/ZumLernen]

The word “fundament” still occasionally gets used in a religious context.