fuck
For me fuck isn't typically used in sentence form, only as a single loudly screamed word.
To me, fuck has become a common variant of "really"
- That really sucks.
- That fucking sucks.
It's typically used for a degree of order more exasperation than the "really" form of the expression. Really is the calm and cold version, while fuck is the emotional, loud, obnoxious version.
Of course there are also its own unique uses that I can't really place a finger on for the proper replacement... like "F' that". Its use always comes down to expressing exasperation or emotional outburst.
I really should try to consider others feelings on the subject and abstain from its use... but it's so engrained that it's difficult to watch among friends. Seems to be a natural part of our language at this point.