There is no general rule. When the interest/coupon is compounding or is reinvested, one should use r^(number of periods). When there is no compounding and the interest is just drawn away, one should use simle multiplication or fractioning. Another thing is the matter of convention - i.e. BEY - bond equivalent yield. When you are determining what is the semi-annual interest rate on a bond which pays r% anuualy, you just divide r by 2 and that is it. Make as many test questions covering that area and the force will come to you.