A. Prefixes are often used to give adjectives a negative or an opposite meaning.
1. ‘in-‘ becomes ‘im-‘ before a root beginning with ‘m’ or ‘p’ (e.g. immature, impatient, impartial, improbable)
2. ‘in-‘ becomes ‘ir-‘ before a word beginning with ‘r’ (e.g. irreplaceable, irreversible)
3. ‘in-‘ becomes ‘il-‘ before a word beginning with ‘l’ (e.g. illegal, illegible, illiterate)
4. The prefix ‘in-‘ does not always have a negative meaning, often it gives the idea of inside or into (e.g. internal, import, insert, income)
B. The prefix ‘un-‘ and ‘dis-‘ can also form the opposites of verbs (e.g. tie/ untie, appear/ disappear)
1. These prefixes are used to reverse the action of the verb (e.g. disagree, disapprove, disbelieve, disconnect, discredit, dislike, dismount, disprove, disqualify, unbend, undo, undress, unfold, unload, unlock, unveil, unwrap, unzip)
C. Other Prefixes in English
1. anti: against (e.g. anti-war, antisocial, antibiotic)
2. auto: of or by oneself (e.g. autograph, auto-pilot, autobiography)
3. bi: two, twice (e.g. bicycle, bilateral, biannual, bilingual)
4, ex: former (e.g. ex-wife, ex-smoker, ex-boss)
5. ex: out of (e.g. extract, exhale, excommunicate)
6. micro: small (e.g. micro-cassette, microwave, microscopic)
7. mis: badly/ wrongly (e.g. misunderstand, mistranslate, misinform)
8. mono: one/ single (e.g. monotonous, monologue, monogamous)
9. multi: many (e.g. multi-national, multi-purpose, multi-media)
10. over: too much (e.g. overdo, overtired, oversleep, overeat)
11. post: after (e.g. postwar, postgraduate, post-impressionist)
12. pre: before (preconceived, pre-war, pre-judge)
13. pro: in favour of (e.g. pro-government, pro-revolutionary)
14. pseudo: false (e.g. pseudo-scientific, pseudo-intellectual)
15. re: again or back (e.g. retype, reread, replace, rewind)
16. semi: half (e.g. semicircular, semi-final, semi-detached)
17. sub: under (e.g. subway, submarine, subdivision)
18. Under: not enough (e.g. underworked, underused, undercooked)
(Source: English Vocabulary in Use, Cambridge University Press)