Fuck a vital swear word that doesn’t have a close equivalent. In English, there are many things that can be said by using the word “fuck” that are difficult-to-impossible to say otherwise.
It can be used in a number of ways:
1) As a verb: Fucking means having certain types of sex that involve penetration. The precise definition is a bit complicated and debatable, but sex that involves penises or dildos going into vaginas or butts is definitely fucking.
Calling an act of sexual intercourse fucking is not an insult, and it doesn’t imply disapproval. Outside of a clinical setting, it is the term most people use. (In a clinical setting, it’s usually considered to be an indication of poor boundaries to call it fucking.)
The word “fucking” can also be a comparatively mild adjective. For example “these fucking apples keep falling off the tree and hitting me on the head” indicates annoyance or frustration or anger about the falling apples. It is not the same as telling the person shaking the tree and causing the apples to fall to fuck off, although it can sometimes be close. (It *does* mean the same thing if it’s uttered in connection with blaming another person. Eg “Stop shaking the tree! It’s making the fucking apples fall on my head!”.
2) When fuck takes both a subject and a direct object, it can be either a neutral way of referring to sex, or it can be a sexualized insult. For instance, “Adam fucked Steve” can mean either “Adam and Steve had sex” or “Adam betrayed/cheated/hurt Steve”.
“Adam fucked Steve over” doesn’t refer to sex; it always means betrayal.
3) Asserting boundaries.
“Fuck off” is the strongest and most forceful way to say “go away and leave me alone” in English. It means “go away, don’t bother me, I’m not going to put up with this”.
“Fuck you” means “you have done a terrible thing, I am angry with you, and I don’t want to engage”
4) “Fuck that” (or the variant “fuck <x>)” is comparatively mild, at least sometimes. It can just mean “I do not approve of this situation” or “I strongly dislike x”. It can also indicate rage. Context and tone matter. If it’s something like “fuck the patriarchy” it usually indicates disapproval but not active rage. If it’s something like “Fuck Steve”, it indicates focused contempt for a particular person.
5) Fuck as a word by itself is just an expletive, and is fairly similar to other expletives. For instance: someone who drops a can on their foot might exclaim “fuck!”.
6) “Fuck yeah” indicates emphatic *approval* of something. For instance “fuck yeah ice skating” is likely to be the title of a blog about how awesome ice skating is. It means approximately the same thing as “hell yeah”, but in a way that’s somewhat more likely to offend people.