Prefixes and suffixes are part of a word group known as affixes. Affixes are word parts that you can attach to words. The word a prefix or suffix is attached to is called the root word, or the base word. The root word carries the central meaning of the entire word, and any affixes attached to the root word modify its meaning. You can use prefixes and suffixes to change the part of speech that a word represents, or to alter its meaning.
Examine the root word. This is the base word to which affixes can be added or removed. Determine the meaning of the root word and the part of speech to which it belongs. For example, the base word in "breakable" is "break." The root word means "to separate into parts with suddenness or violence," and it is a verb.
Determine how you would like to modify the word. Do you want to change the actual meaning of the word, the part of speech it is, or both?
Find prefixes and suffixes that modify the word in the way you want. For example, if you simply want to change its part of speech, you can add the suffix "-able" to get "breakable," which turns it into an adjective while keeping the same general meaning as the root word. If you want to change the meaning of "breakable," you can add the prefix "un-" to get "unbreakable." The prefix "un-" means "not," so adding it to a word gives that word the opposite meaning.