Haint, Taint
Haint is used indiscriminately for haven’t and hasn’t. Taint is used for tisn’t. Their use is indicative of an entire lack of culture.
Isn’t
No one need hesitate to use this word. It is smooth in utterance and contributes much to the freedom and ease of social intercourse. Its equivalent is too stately for colloquial forms of speech, and is often suggestive of pedantry. Compare “Isn’t he an eloquent speaker?” “Isn’t this a beautiful flower?” with “Is not he an eloquent speaker?” “Is this not a beautiful flower?”
Wasn’t
Although not so elegant as the present tense form isn’t, yet the contraction wasn’t is in excellent repute. It is properly used only in the first and third persons singular. No one who makes any pretension to culture would be guilty of saying” You was my neighbor, but you wasn’t my friend,” “We was engaged in trade, and they wasn’t of any use to us.” Say we were or were not, but never wasn’t or wa’nt.
Weren’t
The forms aren’t, and weren’t do not have the sanction of the best speakers and writers, and should be used sparingly, if at all.
Shouldn’t and Wouldn’t
These are frequently used in speech, but are not so common in writing.