NOT A TEACHER
In I found your advertisement in a newspaper, you are not being specific about which newspaper you are talking about. It could be any newspaper, and in this particular conversation that may well be what you want to say. After all, you are really talking about the advertisement, the newspaper in which you saw it might not be of any importance at all.
In I found your advertisement in the newspaper, you are talking about a specific newspaper and assuming that the other person knows which newspaper you mean.
Here's a different example. Imagine you are driving to a city you have not visited before, and you are not sure of the way. You stop your car and say to someone passing by,
I want to go to the city centre. Am I going in the right direction?
You are asking directions to the centre of the city you are in, and hopefully the person tells you how to get there.
But suppose instead of that you said,
I want to go to a city centre. Am I going in the right direction?
This would make no sense, because if you say a city centre, you are not being specific about which city you are talking about. A city centre could mean any city centre in the world – so it would be a bit difficult to tell you how to get there!
Does that help?
Student or Learner