What is a vulgarism and how does it relate to grammar?
According to merriam-webster.com:
A vulgarism is "... a word or expression originated or used chiefly by illiterate persons."
Strunk & White think vulgarisms include wrong word choices:
Wrong: small, home-type hotels
Right: small, homelike hotels
(THE ELEMENTS OF STYLE, p. 62).
Punctuation can also be used in error and result in a vulgarism:
"Question marks . . . have their own vulgarism, an attempt to attribute sarcasm to a word by putting a query in brackets after it: 'We attended a really cultured (?) dinner-party last night, at which the other guests could talk of nothing but film stars and football pools.' This habit should be strangled at birth. . . .
"There is, however, one other misuse of the question mark that deserves more serious comment, namely its intrusion in indirect questions, such as:
He asked me why I was so silent?This is definitely wrong. The original question mark of 'Why are you so silent?' must give way to a full-stop [period] when the question is converted by 'He asked me' into its indirect form, for the sentence as a whole has now become a statement."
(C.V. Carey, Mind the Stop. Pelican Books, 1971)
So, really, any grammar error would be like fingernails on a chalkboard to an astute person of grammatical expertise (not me, by the way).
Does making an word choice error, punctuation error, or grammar error designate you as an illiterate person?