Theory:

A prefix is a group of letters placed before the a word.
Example:
The word “unhappy” consists of the prefix “un-” [which means “not”] combined with the root (or stem) word “happy”; the word “unhappy” means “not happy.”
 
1. de- decode, decrease.
2. dis- disagree, disappear.
3. ex- exhale, explosion.
4. il- illegal, illogical.
A suffix is a group of letters placed after a word.
Example:
The word "flavourless" consists of the root word “flavour” combined with the suffix “-less” [which means “without”]; the word “flavourless” means “having no flavour.”
 
1. ible- reversible, terrible.
2. ily - eerily, happily, lazily.
3. ing - acting, showing.
4. less - friendless, tireless.