then -- tell the long from the short form of the if-statement
if (a<b) then
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 "
if (2<3) print "world"
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.