A prefix is a half word (e g., anti-, ex-, post-, pre-) added to the front of a word to modify its meaning

Prefixes

A prefix is a half word (e.g., anti-, ex-, post-, pre-) added to the front of a word to modify its meaning.

Prefixes contrast with suffixes, which are added to the back of a word. Prefixes and suffixes are known as affixes.

Easy Examples of Prefixes

• 
  microscope
  
• 
  tripod
  
• 
  devalue
  
• 
  re-establish
  

(Sometimes, a prefix is written with a hyphen. More on this to come.)

Got it? Take a quick test.

Real-Life Examples of Prefixes

The four most common prefixes are dis-, in-, re-, and un-. (These account for over 95% of prefixed words.) Here they are in some short quotations.

• 
  To make people disappear, ask them to keep their promises. (American academic Mason Cooley)
  
• 
  It's inconsiderate to expect someone who isn't your boss or your sister-in-law to know who you are. (American author Letitia Baldrige)
  
• 
  Next time I see you, remind me not to talk to you. (Comedian Groucho Marx)
  
• 
  You can't die with an unfinished book. (Author Terry Pratchett)
  

A List of Common Prefixes

Here is a list of common prefixes with some examples:

Prefix	Meaning	Example
a-, an-	without	amoral, atypical
ante-	before	antecedent, antenatal
anti-	against	anti-establishment
auto-	self	autopilot
circum-	around	circumvent
co-	with	co-conspirator, co-pilot
com-, con-	with	companion, contact
contra-	against	contradiction
de-	off	delist, devalue
dis-	not	disappear
en-	put into	enclose, envelop
ex-	out of, former	extract, ex-governor
extra-	beyond, more than	extracurricular
hetero-	different	heterosexual
homo-	same	homonym, homophone
hyper-	over, more	hyperactive
il-, im-, in-, ir-	not, without	illegal, impractical, inconsiderate, irresponsible
in-	into	insert
inter-	between	internet, intersection,
intra-	between	intranet, intravenous
macro-	large	macronutrients
micro-	small	microscope
mono-	one	monocle
non-	not, without	nonentity, nonstarter,
omni-	all, every	omnipresent, omniscient
post-	after	post-mortem
pre-, pro-	before, forward	precede, project
sub-	under	submarine, substandard
syn-	same time	synchronize
super-	above	supervisor, superhuman
trans-	across	transmit
tri-	three	tripod, triceratops
un-	not	undone, unfinished,
uni-	one	unicorn, unilaterally

Why Should I Care about Prefixes?

Here are three noteworthy points related to prefixes.

(Point 1) Are prefixes hyphenated?

By far the most common question with prefixes is whether to use a hyphen or not. In other words, should you write re-consider or reconsider, or anti-aircraft or antiaircraft?

As a general guideline, avoid a hyphen, but if the unhyphenated version is a spelling mistake (let your spellchecker do its job) or looks too unwieldy for you, then use a hyphen.

• 
  proactive, prehistoric, ultraviolet
  

(These words can be written with or without a hyphen, i.e., the hyphenated versions are not spelling mistakes. Remember though, the guiding principle is to avoid the hyphen if your spellchecker lets you.)

• 
  co-opt 
  

(Your spellchecker will bleat at coopt, so you must use a hyphen.)

• 
  Antiaircraft
  

(Antiaircraft is not wrong, but it looks a little unwieldy. If you feel the same way, go for anti-aircraft.)
This overarching guidance will see you through most situations. Below are six points offering a little more detail:

(Rule 1) Use a hyphen with a proper noun.

If your prefix sits before a proper noun, use a hyphen.

• 
  un-British, pro-Nazi
  

(Rule 2) Do not allow the same vowel to double up (unless it's an 'o').

If the prefix ends in the same vowel that the root words starts with, separate them with a hyphen.

• 
  semi-industrious, re-enter, ultra-argumentative
  

However, particularly when the vowel is an 'o,' if you can bear how the word looks without a hyphen and your spellchecker lets it through, then omit the hyphen.

• 
  coordinate, cooperate
  
• 
  coowner 
  

(Your spellchecker, or dictionary, will not allow coowner.)
(Rule 3) You can let different vowels double up.

When a prefix ends in a vowel and the root word starts with a different vowel, it is usual to omit the hyphen.

• 
  proactive, reactivate, semiautonomous
  

(Remember that if your spellchecker doesn't like it or you cannot bear how it looks, go for a hyphen. For many, semiautonomous looks too unwieldy.)
(Rule 4) Use a hyphen with 'ex' and 'self.'

The prefixes 'ex' and 'self' are usually followed by a hyphen.

• 
  ex-husband, self-aware
  

(Rule 5) Eliminate ambiguity every time.

If the unhyphenated version could be confused with a different word, add the hyphen. (This is most common with the prefix 're.')

• 
  re-cover / recover
  

(If there were no hyphen in re-cover, it could be confused with recover, meaning to return to a normal state.)

• 
  re-press / repress
  

(If there were no hyphen in re-press, it could be confused with repress, meaning to subdue with force.)

• 
  pre-date / predate
  

(If there were no hyphen in pre-date, it could be confused with predate, meaning to prey upon.)
(Rule 6) Enjoy the leniency.

Most prefixed words exist in both forms. As you might have noticed from the guidelines above, it is often down to how the writer feels about the word.

• 
  The attack was at night because the anti-government troops did not possess infrared goggles.
  

(Here, the writer did not like the look of antigovernment or infra-red, so chose the versions above. That's fine. Enjoy the leniency.)

• 
  Why is non-hyphenated hyphenated? The irony!
  

(This is a popular internet meme. In fact, nonhyphenated doesn't need a hyphen, but the joke fails without it. Enjoy the leniency.)

(Point 2) Can you add to prefix to reduce your wordcount?

Often a prefix can be added to reduce your wordcount by a word. For example:

• 
  not wanted > unwanted
  
• 
  think again > rethink
  
• 
  not practical > impractical
  

(Point 3) Improve your spelling and vocabulary.

In the entry on affixes, we talk about how you can use your understanding of prefixes to:

• 
  Improve your spelling by breaking a word down into prefixes and suffixes and tackling the parts one at a time (e.g., dis-respect-ful-ly).
  
• 
  Decipher the meaning of a long word (e.g., im-practical = not practical).
  

(A quick aside: Why does inflammable mean flammable? That's crazy.)
из English Grammar Today
Prefixes are letters which we add to the beginning of a word to make a new word with a different meaning. Prefixes can, for example, create a new word opposite in meaning to the word the prefix is attached to. They can also make a word negative or express relations of time, place or manner. Here are some examples:

base word	prefixed word	type of meaning
possible	impossible	opposite
able	unable	opposite/negation
payment	non-payment	negation
war	pre-war	time (before)
terrestrial	extraterrestrial	place (outside of/beyond)
cook	overcook	manner (too much)

I’m sorry I was unable to attend the meeting.
Non-payment of fees could result in a student being asked to leave the course.
Has anyone ever really met an extraterrestrial being? (meaning a being from another planet)
The meat was overcooked and quite tasteless.
The most common prefixes

prefix	meaning	examples
anti-	against/opposed to	anti-government, anti-racist, anti-war
auto-	self	autobiography, automobile
de-	reverse or change	de-classify, decontaminate, demotivate
dis-	reverse or remove	disagree, displeasure, disqualify
down-	reduce or lower	downgrade, downhearted
extra-	beyond	extraordinary, extraterrestrial
hyper-	extreme	hyperactive, hypertension
il-, im-, in-, ir-	not	illegal, impossible, insecure, irregular
inter-	between	interactive, international
mega-	very big, important	megabyte, mega-deal, megaton
mid-	middle	midday, midnight, mid-October
mis-	incorrectly, badly	misaligned, mislead, misspelt
non-	not	non-payment, non-smoking
over-	too much	overcook, overcharge, overrate
out-	go beyond	outdo, out-perform, outrun
post-	after	post-election, post-war
pre-	before	prehistoric, pre-war
pro-	in favour of	pro-communist, pro-democracy
re-	again	reconsider, redo, rewrite
semi-	half	semicircle, semi-retired
sub-	under, below	submarine, sub-Saharan
super-	above, beyond	super-hero, supermodel
tele-	at a distance	television, telepathic
trans-	across	transatlantic, transfer
ultra-	extremely	ultra-compact, ultrasound
un-	remove, reverse, not	undo, unpack, unhappy
under-	less than, beneath	undercook, underestimate
up-	make or move higher	upgrade, uphill