Damn is really not such a bad word. Most people use it a lot in many situations, so you probably won't offend anyone by using it, you will just sound angry or annoyed :) "F*ck" is offensive if said at the wrong times. It has no place in most writing or any formal situation or around children. If someone is your really, really close friend, and would never be offended by your words, then there's not a lot of risk. Beyond that, it's impossible to explain what makes it appropriate or inappropriate. It depends on the situation, the context, the tone etc. However, it's said more than most people probably care to admit. Remember that movies are always painting characters as edgy, street, rough etc, and they often use such language to illustrate that. I don't think it's necessarily representative of how people normally speak (we're surrounded by normally spoken people everyday, people won't pay to hear that :) Since it's not necessary to use such a potentially offensive word, I think it's best to know what some of these phrases mean, but don't use them until you really understand the cultural contexts of their use extremely well. We learn this through experience only.

i think we should give some claps for Steven' answer.nice .

"Fuck" is a very versatile word that can be a noun, verb, adjective etc. depending on the situation. Beside that it can be also very offensive and my advise would be not to use it until you are 100% sure how to apply it. I am 100% sure and I don't use it.

it should only be restricted among friends whose way of expressing emotions are like that... In public or formal gathering, I don't think it is appropriate. :)

たぶん「These phrases are often used in American movies｣のほうがいいですよ。とにかくFuckとDamn itが悪い言葉で普通です。僕にはその単語をいつも使ってるんです。Damn itは日本語でくそのように使えるよ。しかしFuckを説明にくい、多くていろいろな使い方がる。