I know it may seem like a frivolous question, and I’ve always accepted the premise that cursing is a mortal sin, but I’m not sure exactly what Commandments are broken by someone who uses profane language (I’m not talking about swearing or using the Lord’s name in vain). It will be helpful to me in making a case against using foul language.
Again, please keep in mind the three requirements for mortal sin: grave matter, full knowledge, and full and free consent of the will. If any of those three requirements are missing, there is not a mortal sin. So the question is not “Is profanity a mortal sin?” but “Is profanity grave matter?” This holds true with just about every questionable action to trouble the human conscience: We can only determine whether the action is grave matter; only the individual, perhaps in concert with a spiritual director or confessor, can discern whether he has met the other two requirements for mortal sin.
Now, is profanity grave matter? It depends on what you’re considering to be profanity. As you say, blasphemy is an entirely different question. To simplify matters profanity, as opposed to blasphemy or irreverence, is usually considered to be vulgar word choices for certain bodily parts and functions (this is also sometimes referred to as obscenity or coarse language; to simplify matters, we’ll use profanity to encompass all non-blasphemous objectionable language). To the extent that such word choices are used abusively toward other persons or toward God, it may constitute grave matter; to the extent that such word choices are used unthinkingly, for lack of more refined and considerate expressions, it is rude but not necessarily sinful. Rude, in this case, is being used in the sense of either “lacking social graces” or “lacking education in civility.” If one is purposely rude (i.e., ill-mannered, impolite, even though one knows better), it may constitute sin depending on the gravity of the offense.