Comment by elendilm
12 hours ago
Maybe it must be considered from the perspective of the speaker. He may have a particular dentist in mind when he says "the dentist" and maybe the listener not having privy to it may not be relevant.
And he may be saying "a" about the word "nice" instead of about the word "evening".
Note: This is just my opinion and an attempt at trying to make sense of it. Purely subjective take as I usually refrain from making purely opinion based arguments.
No matter how much we try to make objective statements, they are never based on "full, provable truth", and are thus opinions. Conversing with other humans is exchanging opinions based on their own previous knowledge. Thus no need to — when conversing with non-malicious people — state that your opinion is, in fact, your opinion. I appreciate it for what it is, and if I am curious about the basis for it, I will inquire more.
All of the above is in my opinion, but I never state that and assume a good faith reader will take it as such (here for demonstration purposes only :)).
So in short, thanks for your opinion, and I agree this is the largely the basis for choosing between an indefinite and definite article, also opening them up to inquiry ("I am going to the store" — "oh, which one?"; compare to "I am going to a store" — "what are you getting?").