A piece at the front
A prefix is a letter or group of letters added before a word’s base to change its meaning. Add re- to play and replay means play again.
Cambridge’s prefix guide shows how prefixes can express ideas such as not, before and again. They are small pieces with quite different jobs.
Three small changes
Unhappy means not happy. Pre-war places something before a war. Rewrite asks for the writing to be done again.
Try changing the instruction “read the sentence” to “reread the sentence”. Nobody has changed the sentence itself. The prefix changes what you should do with it.
Letters are not enough
Do not chop off a beginning just because it resembles a prefix. The remaining part and the meaning need to make sense too.
Un- in unhappy is a useful example. Treating every word beginning with those two letters in the same way would lead you astray. Next, move to the other end with suffixes.