Conditionals

(if (evenp 7)
    (format t "7 is even~%")
    (format t "7 is odd~%"))
;; => 7 is odd
;;
;;
;;    NIL

p at the end of evenp function indicates that it is a "predicate" (i.e. returns t or nil).
There are no then or else - the second expression is evaluated if the condition is not nil, the third is evaluated if it is nil.

(when (= 0 (mod 8 4))
  (format t "8 is divisible by 4~%")
  (princ "Not, but really!"))
;; => 8 is divisible by 4
;;    Not, but really!
;;
;;    "Not, but really!"

when lets you run multiple expressions given the condition is not nil
if would be perfectly valid for a single expression but when is more idiomatic

(cond ((< 9 0)
       (format t "9 is negative"))
      ((< 9 10)
       (format t "9 has 1 digit"))
      (t
       (format t "9 has multiple digits")))
;; => 9 has 1 digit
;;
;;    NIL

Multiple conditions can be written with cond. The first condition that is not nil will be evaluated and returned.
'Else', as implemented in other languages can be written as the symbol t which always evaluates to t, which is not nil.

(defun what-am-i (i)
  (cond ((stringp i)
         "I am a string!!")
        ((integerp i)
         "I am an integer!!")
        ((listp i)
         "I am a list!!")))
;; => WHAT-AM-I

(what-am-i 1)
;; => "I am an integer!!"
(what-am-i "blah-blah")
;; => "I am a string!!"
(what-am-i (list 1 2 3))
;; => "I am a list!!"
(what-am-i 1/2)
;; => NIL

cond is an expression
There are built-in functions to test the types of variables. As expected (haha) they end in "p"
If none of the conditions match, nil is returned