1. Let's clarify why we add terms when simplifying expressions. 2. When you have like terms, which means terms with the same variable and exponent, you can add or subtract their coefficients. 3. For example, in the expression $3x + 5x$, both terms have the variable $x$ to the first power, so they are like terms. 4. You add the coefficients: $3 + 5 = 8$, so $3x + 5x = 8x$. 5. This works because addition is combining quantities of the same kind. 6. However, you cannot add unlike terms directly, for example, $3x + 5y$ stays as is because $x$ and $y$ are different variables. 7. Grouping terms means collecting like terms together to simplify the expression. 8. So, adding terms happens only when they are like terms, which is why you add their coefficients. 9. If you provide a specific expression, I can show you step-by-step how to group and add terms properly.