I tutor students on writing skills in order to not starve to death in a garret while I write my PhD. Most of the time they have problems with punctuation, argumentation, structure and so on. Most of the time I patiently explain how to do it right, and then they have to work on improving. Sometimes that's hard, like if they have difficulty in writing critically, and they have to really practise. Sometimes it's simple: honestly, the number of times I've explained how to use a semi-colon and the student has said 'Oh, that's quite easy, isn't it? I wish someone had told me that before'.
Every now and then, a student has a problem and I can tell them, 'You already know how to do this; just do it the way you say it'. I love it when that happens. The trouble is, students with poor grammar are used to not trusting their own knowledge. They say I seen it happen, but they must write I saw it happen. They say We don't know the reasons for this but they must write We do not know the reasons for this. They say /ðɛə/ and /ðɛə/ but write there and their. It's a minefield out there.
So I do understand when a student isn't sure about using a and an. It's a simple enough rule, on the face of it: use a before consonsants and an before vowels. But then what about words like university or hour? And abbreviations like MP and USB? If you follow the rule, you end up with something that conflicts with what you feel like it should be. Here, the key is to trust your instincts. Of course, the rule works off the sound of the pronunciation, not the spelling. So University sounds like you- and hour sounds like our. MP sounds like empee and USB sounds like you-ess-bee.
But the clever thing is that I could say to that student, just do it the way you say it and you will always be right. That's quite exciting. A native speaker of English, once they're no longer a very young child, will never ever get that wrong (apart from slips of the tongue and the like - but that's different). They will never be unsure which they should use. It's obviously not innate, as it's a language-specific rule, and children do say Can I have a apple, but I can't recall if it's something you're explicitly taught. I have a feeling it's not. But it is an example of the worry caused by learning that there are rules and that you're probably not doing it right.