roughly, the trigger is going to look something like this, but the DDL asked for in the post above would make it definitive:
we don't know what columns you want to audit, or the PK of the table. i hope you can figure out the changes required to my model here:
CREATE TRIGGER TR_TrackChangesInMain ON MAIN
FOR UPDATE
AS
INSERT INTO AuditTable(ColumnList)
SELECT ColumnList
FROM INSERTED
INNER JOIN DELETED ON INSERTED.ID = DELETED.ID
WHERE ISNULL(INSERTED.start_date,'1900-01-01') <> ISNULL(DELETED.start_date,'1900-01-01')
Lowell