What words, when a single letter is added, reverse their meanings?

language/english/meaning/synonyms/antonym

Problem

What words, when a single letter is added, reverse their meanings?

Exclude words that are obtained by adding an "a-" to the beginning.

Show solution

e: fast -> feast, fiancee -> fiance
h: treat -> threat
r: fiend -> friend
s: he -> she
t: here -> there