One downside to having a background in linguistics is that one is more sensitive to various so-called grammar rules that people regurgitate from their school years.
The majority of linguists probably view these rules the way a doctor would view the four humours. But a more fundamental issue is not that some of the theories have been superseded but that their perpetuation reveals a very unscientific approach to language. It is as if these people are viewing rules of grammar like they would road rules—human inventions that one may disagree with, but which are still, in some sense, what is "correct"—rather than, say, laws of physics that attempt to model observations.
This means that when confronted with data that doesn't match the rules, such people will say the data is wrong ("that isn't correct English") rather than ever consider that maybe it's their rules that need refinement.
Now it is certainly the case that people make mistakes when they speak (and that can be a revealing study in itself) and there is such a thing as good English usage (see the last paragraph) but most linguists focus on modeling the tacit intuitions native speakers have about their language which are very often at odds with the "rules of grammar" learnt at school.
Let me give an example. A common misconception is that English has cases that work in a similar way to Latin. I suspect the origins of this stem from attempts to model English after Latin as if Latin was somehow a better language.
It is easy to see on the surface there might be evidence for a nominative/accusative case distinction in English pronouns. Native speakers will say "I gave the book to him. He gave the money to me." with the intuition that switching "I" and "me" or "he" and "him" would be incorrect. This is a valid observation that a linguist would want to capture in some kind of descriptive rule.
However, when asked "who is it?" native speakers will almost always answer "it's me" rather than "?it's I".
I've had people try to tell me that the latter is "more correct" although they don't say it themselves. If they don't say it, as competent speakers of English, what is their claim to it being correct? Because their high school English teacher told them? Because Latin would use the nominative here?
If you ask a group of people "who wants to sit in the front?" they are far more likely to answer "me" than "?I". And yet they would say "I do" rather than "*me do". If you accuse someone you are probably more likely to say "it was him" not "?it was he". So what's going on doesn't just involve using the nominative case for the subject and accusative case for the object.
The massive 1,800+ page Cambridge Grammar of the English Language gives more great examples "nobody can do it but her/*she", "the only one who objected was me/?I" and (showing photos) "this one here is me/*I at the age of 12".
Things become even more complex in the case of conjunctions. The Cambridge Grammar gives the example of "they invited my partner and I to lunch". They point out that examples like this "are regularly used by a significant proportion of speakers of Standard English, and not generally thought by ordinary speakers to be non-standard". They go on to argue against the prescriptivist use of analogy with "they invited me/*I to lunch" to justify why the use of "I" is incorrect.
The worst kinds of rules are ones that sound almost like superstitions: don't use passives, don't use adverbs, don't split infinitives, don't end sentences with a preposition, don't start a sentence with "however" etc. They may help one adopt a particular style of writing, but they certainly aren't rules of grammar in any scientific sense and are, in most cases, completely arbitrary.
Some of the usage guides these rules are found in are better than others. Strunk and White is full of these arbitrary superstitions. In fact, Professor Geoffrey Pullum, the co-editor of the previously mentioned Cambridge Grammar of the English Language describes Strunk and White as a "horrid little notebook of nonsense" and instead recommends Merriam-Webster's Concise Dictionary of English Usage which I own and agree is much more useful for evidence-based guidelines on subtle differences in usage between words.
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