Some time ago a native English speaker said to me that
That is the tool which I use to edit video
Means the person uses only this tool and
That is a tool which I use to edit video
Means the person uses not only the tool under discussion, which is merely one of them
So I supposed that
The central concern appears to be around a perceived lack of accountability and a belief that youth crime rates in Canada are increasing.
would, therefore, mean there is more that one such belief but that's impossible because I shrank down the number of possible beliefs to just this one (using the part right after the word 'belief'). Please, someone, explain this phenomenon.
Perhaps I'm supposed to get rid of the rest of the sentence if an indefinite article is used and think the following: of all the beliefs in the World there is one, and then name it using a clause. But that's not in alignment with the second example.