That said, "damn" would be acceptable in 90 percent of the situations I can think of. Perhaps she also wants people to learn how to express themselves clearly without needing to use swear words of any type, even mild ones. It's easy to use a swear word.

Is it okay to say damn?

This is because Damn is considered a swear word in English, for historical and religious reasons (as SamBC mentioned earlier). When you want to say a swear word, but don't want to be offensive, people create "Minced Oaths".

Is damn a bad expression?

Frequency: Used to express anger, irritation, contempt, or disappointment. To damn is defined as to condemn to an unhappy life or is used to swear to express anger. An example of to damn is to condemn someone as bad by saying "damn you" after they do something that makes you angry.