IF

Run statements when a condition is true. Single-line and multiline forms are supported. IF cond THEN linenumber is a GOTO when cond is true. A THEN at end of line starts a multiline IF closed by ENDIF (or END IF).

IF condition THEN statement [ELSE statement] IF condition THEN statements ELSEIF condition THEN statements ELSE statements ENDIF

condition - expression; non-zero is true statement - statement to run on the same line

Example:

X = 1
IF X > 0 THEN PRINT "YES" ELSE PRINT "NO"

X = 0
IF X > 0 THEN
  PRINT "YES"
ELSEIF X = 0 THEN
  PRINT "ZERO"
ELSE
  PRINT "NO"
END IF

IF X > 0 THEN Target
PRINT "skipped when X > 0"

Target:

PRINT "here"

Source: docs/help/if.txt inmmcore. On the computer, typeHELP IF.