There is going to be a word (or several) in your native language that your grandparents would never expect to hear you say, and you would get at the very least severe talking-to from your parents if you said it in public as a young child. For those who grew up in english-speaking countries, fuck is one such word. It falls below other words, such as cunt and whore, (which I struggle to type out without asterisks or somesuch self-censoring) but it is not tolerated in children.

There are plenty of less severe words in english that can be used, such as "bloody" or "damn" which roughly equate to how those who speak english as a second language use "fuck"