I really hate grammatical rules, which is a strange thing for an English teacher to say. But I never learnt them, so perhaps my antipathy is justified, or perhaps not! Believe it or not, I only began to learn them when I had to teach them!
But I am convinced that there is no point memorising grammatical rules, because theory and practice are two totally different things. I have students who can answer grammatical quiz questions perfectly, yet when they write paragraphs and essays none of that comes through. Why? Because when we're writing, we're not thinking about the rules. We're thinking of ideas, and how best to convey those ideas... and when you're an ESL (English as a second language) student, it's about all you can do to think of the appropriate vocabulary to articulate your opinions. Grammar becomes secondary.
I didn't realise how influential reading is until I became a teacher. Reading helps one to internalise grammatical rules and sentence structure: it's how you "just know" something is wrong, because it "feels" wrong or "looks" wrong. You may not be able to explain it, but your brain has seen a particular arrangement of words so many times that it just knows that's the way things should be.
So in a way I feel it is pointless to teach grammatical rules, except to give the students some understanding of why certain things are right and others are wrong. But in the end, harping on grammar will get us nowhere. However many times I tell my students that verbs have to change depending on whether the sentence has a singular or plural subject, they still write "Many students likes"... and when I point out, "What did you just do there?" they're like, "Oh yes! Sorry, teacher!" and jump to correct their mistake. They know the rule, but they don't practice it. It's not a part of them... YET.
I also feel that grammatical rules are ridiculously complicated and full of jargon. Coordinating conjunctions, subordinating conjunctions, adverbial clauses, noun clauses, relative clauses, independent clauses, dependent clauses, what?!? If all these things make my eyes glaze over, how much more will my students' eyes do the same! I feel that it's way more important you be able to write good sentences, than to know what all these things are called. Unfortunately, it's difficult to teach how to write a good sentence without using jargon. Especially when you have to explain where to put what and how to arrange all the words and blah.
All of which makes me a rather weird language teacher. But language is not about rules! It's about usage! I don't want my students to get sidetracked by all the details and focused on knowing every single grammatical rule. I want them to be focused on trying to write well. (And of course I'd love them to pass, too. Just sayin'.)