***** NOT A TEACHER *****
(1) I think that the rule is something like this:
Use "a" when you first mention a noun; after the noun has been identified, then
(a) As you know, "a" = one; "the" = that.
(i) A man was running down the street. The man was my uncle. (One man was runnning down the street. That man was my uncle.)
(2) Of course, as in all languages, there are exceptions. For example, you would
say to a colleague:
My car broke down this morning, so I took a taxi. The driver was very honest.
(You do not use "a" (one) because you know that a taxi always has only one
driver. So, of course, you are referring to "the" (that) driver.)
(3) Maybe the use of "human being" is too formal. You might want to use
"person." (Yes, it is better not to use "man," for this might hurt the feelings of the