I'm trying to think of a counter example but the golden rule for a and the is specificity. "A" refers to any thing and "the" refers to a particular thing. No

I'm trying to think of a counter example but the golden rule for a and the is specificity. "A" refers to any thing and "the" refers to a particular thing. Notice I used any thing instead of anything: now that is quite hard to explain. "The book rested on a shelf" and "a book rested on a shelf". You use a or the if you want to draw attention to one or the other or even both. It depends on context. The book I mentioned earlier rests on the top shelf. The top shelf is above another shelf. The other shelf is the middle shelf. If there were lots of sets of shelves then the middle shelf would be a middle shelf, unless I want to call attention to it, then it is the middle shelf. A red toy lorry lies on the middle shelf. The red lorry has a black steering wheel. The steering wheel is broken. There is a logic to all of that - honest!