Forewarn is a verb that implies to warn someone in advance about possible trouble or danger. Antonyms for forewarn include words like reassure, encourage, comfort, and inspire. Someone who reassures another offers reassurance as a noun. When someone encourages another, they are offering hope and support. Offering comfort provides solace, and inspiring means to motivate and encourage.
Some other antonyms for forewarn include faith, trust, and optimism. These words are used when we trust that things will work out and have faith that everything will be alright. Optimism is a state of mind that is always looking for the good in situations and choosing to see things from a positive perspective.