There used to be an advertisement in Australia that contained the statement, “Oils ain’t oils!”.
Now, I know little about oils, but I do know that ‘ain’t’ should not be a member of a child’s vocabulary — or an adult’s for that matter!
It is such a pity that so many supposedly learned individuals employ its usage, virtually without exception.
‘Isn’t’ is used when the subject is singular (one).
Example: Ralph (singular) isn’t coming.
‘Aren’t’ is used when the subject is plural (more than one).
Example: They (plural) aren’t coming.
Betty and James aren’t related.