Prefix

A prefix is placed at the beginning of a word to modify or change its meaning.

This is a list of the most common prefixes in English, together with their basic meaning and some examples. 

Prefix		Meaning		Examples
dis-		negation, removal, expulsion		disadvantage, dismount, disbud, disbar
in-		not, without		infertile, inappropriate, impossible
inter-		between, among		interact, interchange
non-		absence, negation		non-smoker, non-alcoholic
over-		excessively, completely		overconfident, overburdened, overjoyed
pre-		before in time, place, order or importance		pre-adolescent, prelude, precondition
re-		again		repaint, reappraise, reawake
semi-		half, partly		semicircle, semi-conscious
sub-		at a lower position		submarine, subsoil
syn-		in union, acting together		synchronize, symmetry
trans-		across, beyond		transnational, transatlantic
		into a different state		translate
un-		not		unacceptable, unreal, unhappy, unmanned
		reversal or cancellation of action or state		unplug, unmask
under-		not enough		underdeveloped