The noun dam refers to a barrier that holds back water. As a verb, dam means to hold back or confine.
As a verb, damn means to criticize or to condemn as bad or inferior. As an interjection, damn is used to express anger, frustration, or disappointment. As an adjective, damn serves as a shorted form of damned.
(a) "The man could be concealing the fact that his stones were enchanted by black magic, helping to _____ the person who used them." (Piers Anthony, On a Pale Horse)
(b) Waves were crashing against the _____ in front of us, and we were drenched by the wild spray.
(c) For thousands of years, China has wanted to _____ the Yangtze River.