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, ...

Other common prefixes are:
antiagainstanti-war, antisocialmultimanymulti-lingual, multi-purpose
autoof or by oneselfautograph, autobiographyovertoo muchoverdo, ovetired, oversleep
bitwo, twicebicycle, bilingualpostafterpostwar, postgraduate
exformerex-wife, ex-presidentprebeforepre-listening
out ofextract, exhaleproin favour of pro-government
minismallminu-skirtreagain / backretype, reread
misbadly/wronglymisunderstand, misbehavesemihalfsemi-detached, semicircular
monoone/singlemonologue, monotonoussubundersubway, submarine
   undernot enoughunderpaid, undercooked

Adapted from English Vocabulary in Use by McCarthy & O'Dell, CUP 1994.

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