@Sofia
You can say „thuh elevator“ or „thuh onion“ and I`m sure many people do, but you would have to use the glottal stop to avoid running the words together. I`ve frankly never heard anyone run the words together without the stop. I doubt anyone even knows how many people say „thee onion/elevator“ vs „thuh onion/elevator“ and I`m quite sure few Americans even know or hear the difference, it´s that subtle.
This is one of those things native speakers are never taught (except for maybe in speech therapy class). Native speakers are not taught pronunciation. In fact, they know it before they even get to school. Only when they get to school and speech impediments are noticed (like a lisp) are they taught to overcome it.
I have no idea when I say „thee onion“ vs. „thuh onion“ and in my entire life have never even thought about it until now. I doubt it`s a hard and fast rule of any sort.
@Neal
If you had the lisp as a little kid, which is common, then you would have, and should have, been taken aside and given speech therapy classes for that. I don`t know what the percentage is of children with this problem is, but it`s certainly not uncommon. I never had any speech impediments as a child, but I know of a few kids who did. My cousin, who was my age, had a speech problem of some kind (I`ve forgotten exactly now what it was), and I think it was probably the lisp as you mentioned, but after speech therapy in school, he got over it too. I wonder if that´s the only sound the few kids with speech impediments have in English, the lisp. With Castellon Spanish or Icelandic, since at least Icelandic has the TH sound in just about every word and sometimes it appears multiple times in one word, I have to wonder if their children have this same problem.
The only thing I recall is my mom constantly correcting my younger sister to not say „It happened on accident“ and instead to say, „it happened BY accident.“ I assume my sister just picked the „on accident“ up off the street. A lot to do over one lousy preposition.
I never had pronunciation or language problems of any kind, for some reason, at school. Languages just came to me easily (even the dreaded high school Spanish).
So as to English grammar, sure it was taught it and the „rules“ were quickly forgotten once the class was over. I was never taught pronunciation. Like Noam Chomsky says, „If someone has to teach you the grammar of your own language, the odds are pretty good that what you are being taught is false. Otherwise you wouldn`t have to be taught it. No one teaches you your own language, any more than they teach you how to see. It´s something that just grows in your head....children already speak their own language before they even get to school.“