A, an and the in use (What the writer assumes the reader knows).

Examples: I got an A on the test.

More than one A was given, or at least the writer thinks so, and the writer got one of them. The writer assumes the reader knows which test.

I got the A on the test.

Only one A was given on the test, and the writer got it. The writer assumes the reader knows which test.

The car is in the parking lot.

Both reader and writer know which car and which parking lot.

A person could be in danger here.

Both reader and writer do not know the specific person.