Comment by hackyhacky

> [I learned it] "at school" - I learned it at school, I'm not specifying which school

> [I learned it] "at the school" - I learned it at school, the one that was referenced

That's not the distinction that the OP is making. If you say "I am at school", it means you are a student, you attend school. If you say "I am at THE school", it means you are physically at the school building, without specifying whether you are a student. Both of those options are different from saying "at a school." See also "prison", "work", "bed". Similar distinction (in UK English, not so much American English) with "in hospital" vs "in the hospital."

As was pointed out, "I go to the dentist every year" doesn't necessarily mean a particular dentist; it just means some dentist, but we use the definite article anyway, because I guess we consider all dentists interchangeable? We don't use definite articles even when we are referring to a specific thing in specific phrases, such as "I went to Florida by [the] car."

Then there's highly idiomatic use of articles with proper nouns. We say "Buckingham Palace" [no article] but "The Washington Monument" [definite article]. We talk about "The Rockies" [definite article] but "Mount Fuji" [no article]. "Germany" but "The United States." Sometimes we use the definite article when referring to to racial or ethnic groups, and sometimes we don't, and that choice can have complicated social implications ("The Blacks", anyone?).

In short, if a native English speaker tells you that article use is straightforward, then they simply have not considered all the cases.