You need a common ground.
Combining fractions requires matching denominators.
Without a common denominator, you're adding apples to oranges.
Butterfly method or LCD (Lowest Common Denominator).
Plain English: You cannot add slices of pizza if they are cut into different sizes. You have to cut them so they match sizes first. You can't add 'Thirds' to 'Fourths' until they both become 'Twelfths'.
Real-world example: Time management. You work 1/2 an hour, then 1/4 of an hour. Total time worked?
Adding the denominators! 1/2 + 1/2 is NOT 2/4. It is 2/2 (1 Whole).