then -- tell the long from the short form of the if-statement

 
if (a<b) then 
  ... 
endif 

The keyword then is part of the if-statement; please see there for further explanations. However, not every if-statement requires the keyword then: If the keyword then is present, the if-clause may extend over more than one line, and the keyword endif is required to end it. If the keyword then is not present, the if-statement extends up to the end of the line, and any endif would be an error.

if (1<2) then 
	print "Hello "
endif 

if (2<3) print "world" 
if (2<1) 
	print "!" 

This example prints Hello world. Note, that no exclamation mark (!) is printed, which might come as a surprise and may be changed in future versions of yab.