SELECT a.field1,a.field2,a.field3,b.field4 FROM modacad a,crsfinal b ;
WHERE a.mainkey=b.mainkey
INTO CURSOR crsmatch

you have to write your field names insted of "a.field1,a.field2,a.field3,b.field4"

Your mistake in your example you use one static key from a table and your cursor created from another table.