In English, the prefix, or letters before a word, can give a clue to its meaning.  

For example, RE before a word means ‘again’.  Review is to view again. React is to act again.  Redo is to do again.

CO before a word means ‘with’.  Cooperate is to operate with others.

EX before a word means ‘former’.  An ex-husband is a former husband.

MINI before a word means ‘small’.  The minivan is a small van.

And the prefixes, IL, IM, IN, IR, UN before a word mean ‘not’ or ‘without’.

Illegal is not legal.  

Immoral is not moral.  

Inconsiderate is not considerate.  

Irresponsible is not responsible.

Unequal is not equal.

PRE before a word means ‘before’.  

Prefixes are the letters before a word.