decisions
There are types of decisions, two-way, many-way, plus the already discussed computed decisions.

IF statement
The IF statement is the most basic decision mechanism, providing for a simple choice between two possibilities.  In its simplest form, without the ELSE part, the IF statement executes a block of statements if a variable or expression is TRUE (non-zero), or skips it if it is FALSE (zero).  In its full form, with the ELSE part, the IF statement executes the first block of statements if a variable or expression is TRUE, or the second block if it is FALSE.

The following example shows the two kinds of 2-way decisions that can be built with IF statements.

FUNCTION IfDemo(x, y)
  IF (x < y) THEN x = y : RETURN (-1)
  IF (x > y) THEN RETURN (0) ELSE y = x : RETURN (+1)
END FUNCTION

In the first IF statement, x=y and RETURN(-1) are executed if (x<y) is TRUE (x is less than y), otherwise nothing is executed and program execution continues with the next source line.

In the second IF statement, RETURN(0) is executed if (x>y) is TRUE (x is greater than y), otherwise y=x : RETURN(+1) executes.

If an executable statement follows THEN on the same source line, a one-line IF statement is assumed and the end-of-line serves as an implicit END IF.

Otherwise a multi-line block structured IF is assumed and an explicit END IF is required to end it, as in the following example:

FUNCTION IfDemo(x, y)
'
  IF (x < y) THEN
    x = y
    RETURN (-1)
  END IF
'
  IF (x > y) THEN
    RETURN (0)
  ELSE
    y = x
    RETURN (+1)
  END IF
END FUNCTION

Simple IF statements, like IF (x<y) THEN x=y, are normally written on one line.   When more than one statement follows THEN, or when an ELSE section follows the THEN section, multiline form is generally more readable and less error prone.