The expression for "12 more than $a$" is:

$a + 12$

This represents adding 12 to $a$.

Would you like further clarification or have any questions about this? Here are some related questions you may find interesting:

1. How do you represent "10 less than $x$" as an expression?
2. How would you express the phrase "3 times $b$ decreased by 5" mathematically?
3. What does the expression $2x + 7$ represent?
4. How would you write an expression for "half of $n$ added to 4"?
5. What does the phrase "the sum of $x$ and $y$" look like algebraically?

Tip: When writing expressions for phrases, focus on keywords like "more," "less," "times," and "divided" to determine the operation needed.