Negative prefixes before adjectives
We can form the opposite of many adjectives or give the negative meaning by adding a negative prefix. (a prefix is a syllable that goes before a word)
 
 Have a look at these examples:
                     Happy ---> unhappy
                     Fair---> unfair
                     Friendly ---> unfriendly
                     Important ---> unimportant
                     Pleasant ---> unpleasant
                     Popular ---> unpopular
                     Prepared---> unprepared
 
These are the most common negative prefixes used with adjectives:

dis-	il-	im-	in-	ir-	un-
disrespectful	illegitimate	impossible	indecent	irrelevant	unreasonable
dissatisfied	illogical	immature	incapable	irregular	unfortunate

Other negative prefixes are:

a-	anti-	countrer-	mal-	non-
amoral	antisocial	counterproductive	malcontent	non-violent
asexual	anti-aircraft	counterfeit	malnourished	non-profit

The prefixes im- il- and ir- are in fact a variety of in-:
·                     im- is used before words beginning with m or p: impersonal, immortal.
·                     il- is used before words beginning with l: illegal.
·                     ir- is used with words beginning with r: irregular.