If you want to hear the news, you can read a paper. (We use "a paper" to refer to a specific newspaper.)

I thought there was somebody in the house because there was a light on inside. (We use "a light" to refer to a specific light.)

Light comes from the sun. (Here, "light" is a general, uncountable noun.)

It's very difficult to find a job at the moment. (We use "a job" to refer to a specific job.)

We were very unfortunate. We had bad luck. (Here, "bad luck" is a general, uncountable noun.)