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if takes a test, then-branch, optional else:

(if (> x 0) "positive" "non-positive")

For multi-statement branches, use progn:

(if (> x 0)
    (progn (format t "big~%") (do-thing))
    (format t "small~%"))

when is if with no else and implicit progn:

(when (= status :ready)
  (log "starting")
  (start))

cond for multiple branches:

(cond
  ((>= score 90) "A")
  ((>= score 80) "B")
  ((>= score 70) "C")
  (t "F"))                      ; t catches the rest

Falsy values: only nil (also written as ()). Everything else is truthy, including 0 and "".

Keywords start with : and are common as enum-like values: :red :green :blue.