A hardware description language looks like a programming language and does something quite different. What is the essential difference, and why does it catch newcomers out?

A. Because HDL describes hardware that exists all at once, in parallel.

Software executes one instruction after another; hardware is all present at once, so two concurrent assignments are two circuits and writing them in the other order changes nothing. The characteristic trap is that a for loop unrolls, a thousand iterations generates a thousand adders. A conditional becomes a multiplexer, because both branches exist physically.