Only when being paid to, or when someone falls afoul of Gaudere’s Law.
I’m an editor, so mentally correcting people’s grammar is instinctive for me. But I’ve learned to be very careful about whom I actually correct out loud. I would never correct a stranger’s grammar, because it would make me look snobby.
On the other hand, I would edit my last boyfriend’s love letters. 
No I don’t correct grammar. IMO, it would be just as rude as telling a stranger at another table that they shouldn’t be using a salad fork to eat their pasta. The “error” corrected by the OP is just as minor and inconsequential as the “error” of eating with the wrong fork, and it’s petty to let something like that bother you.
dragoncat, how would you respond to someone telling how to speak? If it would bother you, if even just a little bit, then that should give you an idea of how other people feel when you correct them.
I do depending on the relationship I have with the person - but these are all people who speak English as a second language. Or canbe thweir third or fourth language…
Or even more.
Sweet Jesus, no. I teach freshman composition. I spend MORE than enough time correcting grammar as it is; I’m not about to do it without getting paid for it.
Yep. I don’t correct people for free.
No way. I accept all forms of friendly colloquial speech. If I’m in a friendly conversation with someone I don’t need to fix their grammar. If I’m in a business conversation with someone, I’d better NOT fix their grammar!
My Mom is like the Adolf of all grammer nazis.
I used to purposely say f’d up stuff just to get her going.
For instance, one time I was trying to decide what I wanted to take for my sophomore electives. I was considering taking spanish, so I ran it by my Mom like this:
Me: “Mom, I want to learn to speak Mexican.”
Her: :mad::mad:“SHAKES!! PEOPLE DON’T LEARN HOW TO SPEAK MEXICAN!! THEY LEARN HOW TO SPEAK SPANISH!!”
Me:
But even she wouldn’t correct a total stranger.
Shakes, are you one of my kids? They do this to me, now that they’re old enough to get away with it. Mostly it’s the may/can thing, or “me and Sally”, and then I’ll look over to see the kid just grinning, waiting for me to say something.
But, as you said, even I don’t do that to strangers. Or even my kids’ peers. (Anymore)
John killed who?
I feel exactly as expressed by Fretful Porpentine
And I might go further and suggest one study prescriptivism and descriptivism before showing any emotional about grammar in any context.
I’m not twickster, but IME people who make a specific mistake speaking will make it in writing as well. Teaching “my writers” to use correct grammar and make sure their audience is understanding them correctly (pronunciation, tone, emphasis, rythm, register) does wonders for their writing. Once people learn to “step off the big dictionary” when they’re speaking with a mechanic, they’re a lot less likely to write courses for mechanics as if they were for CPAs; once they learn to avoid assuming that their audience will share their hobbies they’ll also be better at coming up with examples for the courses; if they don’t know when to use in/on (English) or ser/estar (Spanish) verbally, they’ll mix them up in writing as well.
I don’t correct strangers or clients, but I do correct anybody whose writings I’ll have to edit and people that I’m friendly with and whom I know are working on learning the language. I do my best to avoid overwhelming them or bogging down the conversation. And I always explain the “whys” of any correction, whether they are on written or spoken material.
Okay, I stand corrected. Of course a pronoun can be an object. But I don’t see how it can be an object and a subject at the same time. And “can I” … is perfectly correct, just perhaps less polite than “may I.”
Well my parents were both English teachers, and believed that everyone in the world wanted their mistakes pointed out. I do, indeed, find it exceedingly annoying. So I guess you all are right that the next 3,000 times I hear that phrase I say nothing. But I don’t spend all day using the same incorrect phrase that makes me sound like a cretin. I can’t imagine anyone using an expression “can I help who’s next” in everyday conversation. And I bet Chomsky has an explanation for that.
No, that’s perfectly good grammar. He meant “oh no, the next customer is that rude, pushy, know-all. I really wish it wasn’t, but can I help who’s next?” :rolleyes:
Neither did the person whose grammar you corrected.
In the example you presented it’s not the case. “Who” is the subject of the noun clause “who [is] next.” The whole noun clause “who is next” is the direct object. Diagrammatically, it would be (un-interrogacized):
I (subject)
can help (verb)
who is next (whole clause is the direct object).
Within that clause,
who (subject)
is (verb)
next (predicate adjective).
And your parents were English teachers. . . :dubious:
There is one that affects me like fingernails screeching down a blackboard. It’s when someone says something like “Me and Joe went to the movies”. I will correct that. The rest of the time, I’ll just shake my head.
Isn’t the important point that you knew what they meant? People also speak colloquially so by rephrasing your approach (Gosh, I haven’t heard a question phrased that way before) accompanied by a dazzling smile you may have learned something new yourself… Just a thought 
Well, that isn’t necessarily the best way to describe the situation. But what the word “who” is doing in that example is exactly analogous to the role played in “The prize will be awarded to whoever eats the most pies” by the word “whoever”; that is, “whoever” is the subject of the element “whoever eats the most pies”, which is itself employable as a noun phrase, in this case as the object of the overarching sentence. This construction is called a fused relative (also known as a nominal relative), and is perfectly grammatical and ubiquitous in English. It’s like a special, more compact variant of the construction in “Can I help the person who is next?” or “The prize who will be awarded to the person who eats the most pies” [to which you might again object that something is being illegitimately used as both a subject and an object, though to raise such an objection would again be misguided].
The only unusual thing in “Can I help who’s next?” is the use of the word “who” to head the fused relative (a role generally limited to “what” and the -ever words (“whoever”, “whatever”, etc.) in modern English). But there’s nothing fundamentally wrong with some words functioning in this way, and never has been.
Now, if you like, you can go look up a long, detailed account of the grammar of modern English written by professional linguists, acquaint yourself with their analysis, study their proposed formalizations, etc., and eventually admit “Ok, they say it’s alright, so I’ll accept it.” But, the really key point is that you shouldn’t bother: you shouldn’t need an explanation like this post before you are willing to countenance the cashier’s actions. People don’t need to engage in academic study of the technical structure of their native language in order to be permitted to choose how they speak. That’s having the tail wag the dog. People speak however they speak, and the only legitimate role of a formal grammar is as a tool to aid scientific understanding of the manner in which they speak. You don’t need to consciously learn some rules in order to be allowed to make communicative sounds with your mouth, lest you accidentally commit some crime of producing sounds in an unsanctioned manner; rather, you are perfectly morally free to speak naturally in whatever particular idiolectal manner that is, almost certainly very similar to that of the general speech community you are a part of and through which you learnt your native language by immersive osmosis, and the legitimate “rules”, like physical theories, are only devised after the fact by scholars who wish to model this behavior you already engage in of your own accord.