Here's why:
* Prefixes are word parts added to the beginning of a word to change its meaning (e.g., "un" in "unhappy").
* Suffixes are word parts added to the end of a word to change its meaning or grammatical function (e.g., "-ing" in "walking").
"Forever" is a word formed by combining the words "for" and "ever". It's not formed by adding a prefix or suffix to a base word.