What are the most fundamental topics in economics?

By the most fundamental I mean the most important topics which if learned, can make everything else make sense easily because all the other topics are built on top of these. And please don't respond with something like "Supply & Demand" as that would be too vague.

Fundamental/core/principles – call it whatever you want. Also, please tell me how you know it? The answer could be anything like maybe you are a undergrad/grad student or professor or maybe you have self studied.

Thank You in advance.