Prefixes are often used to give adjectives a negative meaning. The most common adjective prefixes are un-, in- and dis-: uncomfortable, inconvenient, dissimilar, ...

in- becomes im- before a root beginning with 'm' or 'p' (immature, impatient), ir- before a word beginning with 'r' (irregular) and il- before a word beginning with 'l' (illegal, illiterate).

in- does not always have a negative meaning; it often gives the idea of inside or into: internal, import, ...

un- and dis- can also form the opposites of verbs: appear/disappear, load/unload, ...