In English, a prefix is a few letters placed at the beginning of a word to modify its meaning. For instance dis- is a prefix and placing it in front of "appear" makes "disappear".You wouldn't think a few letters could be so powerful, but prefixes are. Often they turn words into their opposites, as in the case of "appear" and "disappear". It might not always be obvious how the prefix relates to the root word, however. Dis- means a negation. But what does it negate in the case of "disturb"? In this case, the prefix intensifies the root (which is related to turmoil or turbulence). Sometimes the root word changes its meaning so it is no longer as clear how the prefix is operating, as in the case of "disabuse", which means "to undeceive". And what about "dismay"? Can it be the opposite of "may"? Amazingly, it is! But this is only true for the older sense of "may" as having the power to do something, which is much stronger than our current sense of it as a word connected to possibility or having permission.
Try this quiz on the subject to help you get the hang of prefixes.