When I was in primary school, we learned English grammar. Seems like I'm in the minority in that regard when it comes to the schooling of most my colleagues.
I was a kid in the USA in the 1970's. From what I've heard anecdotaly, any American younger than I didn't get taught grammar in English class as an elementary school student.
My UK co-workers have told me the same thing, i.e., their English classes were about esoteric things like recognizing meaning in writings, English lit history and the likes. There weren't any polemics and rules taught after about 1978.
You don't end a sentence in a preposition. Yeah, this was one of the rules I was taught as a kid, but even then, it was taught half-heartedly. English is always changing. Controversial sentence structures become acceptable which had once been proscribed. Nowadays, it's perfectly acceptable to end your sentence a preposition with.
Another rule I learned as a kid was: you should never start a sentence with a conjunction.
And, but, or and so were forbidden as the first word of a sentence when I was 7 years old. That was almost 40 years ago now, and any reading of media today is chock full of sentences starting with those forbidden words.
Used judiciously by wordsmiths of high caliber, these little words add a stylistic brevity to prose. Why write however when you can just write but? So, the rule against conjunctions starting sentences has fallen by the wayside when it comes to 21st Century professional writing.
Should we use the standards we hold our best writers to when marking the writings of our non-NES students?
When I'm marking the writing of an adult ESL student, who writes about as well as I did when I was 7 years old, and they begin a sentence with a preposition, I can't help but remember what I was taught as a kid about English: Don't Start a Sentence With And, But or So!
Many people who are thoughtful about English today consider that rule about conjunctions at the start of a sentence to be the same as the old rule about prepositions at the end of one. It's now okay, they tell us, to start a sentence with a conjunction. They point to the prevalance of this tendency in modern media as justification.
Here's the thing. Stylistically, an expert user of English can mold the language to fit their needs. Expression becomes more important than polemics, and rules go out the window.
For an ESL learner, however, these rules are important because they give a writer a universally comprehensible springboard from which to start. You can only break the rules for stylistic reasons when you know the language well enough to know when you're breaking the rules...
People are lazy. For an ESL student, when it comes to writing, if they can get away with starting a sentence with 'And', which is one of the first words they learn in English, they're going to do so. If 'And' is okay (which it isn't) then the students aren't inclined to learn the subtle differences between phrases like in addition, also, furthermore, on top of that, and (the dreaded) moreover .
It all just becomes AND... Why should they try to learn and use more than AND if it's okay? It's the same with the other conjunctions; reliance on the basic forms retards students' ability to use more distinguished forms.
Don't end a sentence a preposition with... I understand why we no longer follow this rule.
And don't start a sentence with a conjunction: I teach my students this rule while recognizing that many advanced English writers break it. I tell my students they're not advanced.
I don't say it in so many words, but I think that you have to earn the right to start a sentence with a conjunction.
For either NES or non-NES writers, it has to flow out of a command of using a variety of sentence structures effectively.
As an ESL teacher, I'm not dealing with writers who have that kind of command, and so, for them, the rule I learned in 2nd grade English class definitely still applies: don't start a sentence with and, but or so.