Prefix is a group of letters 

added to the beginning of 

the words.

happy, unhappy

Unhappy means not happy.

lucky, unlucky

Unlucky means not lucky.

like, dislike

Dislike means 

the opposite of like.

appear, disappear

Disappear means 

the opposite of appear.

fill, refill

Refill means to fill again.

paint, repaint

Repaint means paint again.

write, rewrite

Rewrite means to write again.

do, redo

Redo means to do again.