Addition Rules: Addition can only take place when the terms being added are the "SAME".
When adding decimals, the decimals must line up, be in the "same"
Keep the decimal in the same place.
When adding fractions, the denominators must be the "same".
Keep the denominator the same.
When adding integers, the numbers must have the "same" signs.
Keep the same sign.
When adding variables, the variables must be the "same", that includes that their exponents must also be the "same", but the coefficients may be different. Keep the variable part the same.
What was the "same" will stay the "same" after the addition has been computed.