Question

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

Answer 1

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

Answer 2

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.