This article is from the Puzzles FAQ, by Chris Cole firstname.lastname@example.org and Matthew Daly email@example.com with numerous contributions by others.
What words, when a single letter is added, reverse their meanings?
Exclude words that are obtained by adding an "a-" to the beginning.
e: fast -> feast, fiancee -> fiance
h: treat -> threat
r: fiend -> friend
s: he -> she
t: here -> there