Define & If

define binds a symbol to a value:

(define x 42)
(define name "Alice")
(define area (* 3.14 r r))   ; r must be defined

It's a SPECIAL FORM: the first argument (the symbol name) is NOT evaluated — that would try to look up x, which doesn't exist yet.

if is also special: only evaluate the chosen branch.

(if (< x 0)
    (- x)        ; evaluated only if x < 0
    x)           ; evaluated only if x >= 0

Lisp's "true" is anything except false and nil in Scheme; just false in Common Lisp; nil and false in Clojure. We'll use Python False and 0 as falsey to match Python conventions.

Special forms list grows: define, if, lambda, quote, set!, begin, let. Everything else is a function call.