Some words are formed with the help of a prefix. A prefix is a syllable placed at the beginning of a word to qualify its meaning and form a new word. Most of the prefixes in English have been borrowed from Latin and Greek. A few are from old English. A list of the most important prefixes is given below.

Ambi (on both sides): ambidextrous, ambivalent, ambiguous

Anti (against): antidote, anti-social, anti-national

Auto (self): autobiography, autocrat, automatic, autograph

Bene (well): benediction, benevolent, benefit

Bi (two, twice): bilateral, bigamy, biweekly, bisect

Circum (around): circumference, circumnavigation, circumstance, circumscribe

Co (with): cooperate, coordinate, co-existence, coalition

Contra, counter (against): contradict, contravene, controversy, counteract, counterbalance

De (down): dethrone, defame, demoralize, denationalize

Demi (half): demigod

Dis (apart): disconnect, discord, disorganize, dispassionate

Extra (beyond): extraordinary, extravagant, extraterritorial

Hyper (over): hypercritical, hypersensitive, hypertension

Mono (alone): monogamy, monopoly, monotheism, monotony

Non (not): nonviolent, nonsense

Omni (all): omnipresent, omnipotent, omnivorous, omniscient

Poly (many): polygamy, polytheism, polysyllabic

Post (after): post-war, post-script, post-mortem

Pre (before): pre-war, precaution, premature, prefix, prehistoric

Pro (for): pro-American, pro-communist

Pseudo (false): pseudo-classic, pseudonym

Re (again): return, refund, revive, research

Semi (half): semicolon, semi-final, semi-formal, semi-official

Sub (under): subconscious, sub-human, sub-caste, subordinate, subjugate

Super (over): superman, supernatural, superhuman, supervise, surpass, surcharge, surplus

Trans (across): trans-Atlantic, transcribe, transcend

Tri (three): tricolor, trinity, triangle

Ultra (beyond): ultramodern, ultraviolet

Un (not): unnatural, unbalanced