A prefix is a group of letters added to the beginning of a word to form a new word with a different meaning. (By contrast, a group of letters attached to the end of a word is called a ‘suffix’.)

It is important to know the meaning of each prefix in order to understand the meaning of the new word.
For example, the prefix ‘semi‘ added to ‘circle‘ forms a new word: semicircle (= half a circle)

Note: There is no absolute rule concerning the use of hyphens.
Here are some examples of commonly-used prefixes, with their meaning.

PREFIXES IN ENGLISH

ante- (before)anti- (against)auto- (self, same)de- (down, reverse)
antecedent
antechamber
antenatal
antepenultimate
antibiotic
anticoagulant
anti-pollution
antiseptic
autobiography
autofocus
autograph
autoimmune
deactivate
decompress
degrade
devalue
dis- (opposite of)extra- (beyond)hyper- (extreme)il-, im-, in-, ir- (not)
disagree
disappear
dishonest
dislike
extracurricular
extrajudicial
extra-terrestriel
extraordinary
hyperactive
hypermarket
hypertension
hyperventilate
illegal
immoral
incomplete
irrelevant
inter-(between, among)intra-, intro-(within, inside)macro- (large)mega- (very big)
intercultural
intermingle
international
interstate
intracerebral
intramuscular
introspection
introvert
macrobiotic
macrocosm
macroeconomics
macroorganism
megabyte
megaphone
megapixel
megawatt
micro- (very small)mini- (small/short)mis- (incorrect)mono- (one, alone)
micro-biology
microchip
microscope
microwave
minibar
minibus
minischool
miniskirt
miscalculate
misinterpret
mispronounce
misspell
monochrome
monogram
monologue
monotheist
multi- (many)omni- (all, every)over- (too much)out- (go beyond)
multimillionaire
multinational
multistorey
multitask
omnibus
omnipotent
omnipresent
omnivore
overburden
overcharge
overcrowded
overrated
outdated
outgrow
outrun
outweigh
poly- (many)post- (after)pre- (before)pro- (in favour of)
polygamy
polygon
polyglot
polyphonic
postgraduate
postmortem
postnatal
postscript
preheat
prepack
prepay
preview
pro-communist
pro-government
pro-life
pro-nuclear
semi- (half)sub- (below)super- (above)trans- (across, through)
semi-annual
semicircle
semi-conscious
semi-detached
subcontract
submarine
subtitle
substandard
superintendent
superstar
supernatural
superimpose
transalpine
transatlantic
transgenetic
transplanetary
un-(not, opposite of)under- (less than)uni- (one, single)up (higher, better)
unable
unhappy
unimportant
untidy
underage
underestimate
underground
understatement
unicycle
uniformity
unilateral
universal
update
upgrade
uphill
upmarket