An IF statement has the form:
IF B_1 THEN S_1
ELSIF B_2 THEN S_2
...
ELSIF B_n THEN S_n
ELSE S_0
END
where the B's are boolean expressions and the S's are statements. The "ELSE S_0" and each "ELSIF B_i THEN S_i" are optional.
The statement evaluates the B's in order until some B_i evaluates to TRUE, and then executes S_i. If none of the expressions evaluates to TRUE and "ELSE S_0" is present, S_0 is executed. If none of the expressions evaluates to TRUE and "ELSE S_0" is absent, the statement is a no-op (except for any side-effects of the B's).