The two most common prefixes are un, which means not, and re, which means the opposite or reverse of something. These prefixes are useful for spellers because they occur frequently and their meanings are easily understood.
Some other common prefixes are in, which means not or into, as in the word incomplete; dis, which means not or apart, as in the word disharmony; and ab, which means away from, as in the word abnormal. A few others include ante, which means before, as in the word antecedent; and anti, which means against or opposite of, as in the word anticlimax.
When adding prefixes to a word, the spelling of the base word never changes. For example, happy becomes unhappy and code becomes recode. Also, spellings of the prefixes themselves never change.
Double letters sometimes occur in words with a prefix. For example, the prefix il is added to the word logical to make the word illogical, and the prefix un is added to the word necessary to make unnecessary. Hyphens usually separate two a's or two i's, as in anti-inflammatory or ultra-ambitious; a hyphen is sometimes inserted between two o's or two e's for improved readability as well. Other instances when a hyphen is needed include after the prefix self-, as in self-explanatory, and the prefix ex-, as in ex-wife. A hyphen also follows any prefix before a proper noun, as in anti-American.