When I add or subtract fractions, first I must convert the

You must convert either one or both fractions so they can have common denominators

Whenever you add or subtract fractions you have to have the denominators the same so that the numerator is of the same value system.

For example you have 8/16 - 8/32

If you were you to subtract it without changing denominators to the same it would be equal to zero but when you change them to the same denominator

16/32 - 8/32 then the answer would be 8/32.