Learn English – How to tell someone I had a great time the last time I saw them

Jack's mother will be so relieved that none of the other stories about Anna are true that she will be less upset when she finds out Anna is not Irish.

The context allows one to assume that Jack's mother has her heart set upon any woman Jack ends up with being Irish. Since Anna presumably isn't, Jack has told her all sorts of horrible untruths about Anna, painting her as the worst sort of person imaginable. The supposed result of her finding out that Anna is nowhere near as bad as she had been described is that she will be so relieved, she will overlook her non-Irishness. The joke could be told of any nationality or religion; the Irishness is incidental

Normative instruction in things like grammar, spelling, punctuation, and syntax accomplish several goals that further educational needs by standardizing on (usually written) language.

When your child says, “Mommy, I taked out the trash already,” you naturally correct her usage to the standard took out the trash. This helps her function in a society that relies upon standard English for communication. You are being prescriptive in your usage. You do this for reasons not so different from a publishing house who has a formal, written style-guide to help its editors and authors provide consistency in their treatment of English.

But what you are doing, what the publishing house is doing, is not linguistics. You are establishing a normative standard. Linguistics is about describing how language works; it makes no sense to talk about “prescriptive linguistics”.

That doesn’t mean that prescription is automatically bad. It isn’t. Sometimes, as shown above, it is both necessary and good. It just isn’t good for describing how people use language, because a prescription can never be a description. Each has its own purpose.