The first step in understanding how a/an or the are used before nouns, is to realise that the default is not to use them at all – just to use the noun alone.Just as the bare verb, known as the Simple Present, is the default form for verbs and needs no justification for its use, the bare noun – no preceding determiners – is the default form of nouns in English. It seems common sense that if a speaker uses a word, they should know why they do so, but many learners have been led to believe that a determiner is required before every noun and so put one in, more or less at random.
It’s very easy to express oneself at length without using any determiners before the nouns. For example:
“Jean lives in Paris which, as you know, is in France. He’s young and goes to university where he studies hard. He likes to enjoy himself too. He plays football, listens to music and reads science fiction among other hobbies. In spite of recent events, he’s not afraid of terrorist attacks. He goes out a lot to eat in restaurants, attend concerts and visit friends. He doesn’t often go to bed early.”
In traditional grammar books you will find long lists explaining various reasons for the “absence” of determiners in each case in the text above. Don’t clutter up your mind with them! It’s very difficult and rather ridiculous to justify a negative. If asked why you live where you do, is it normal to give reasons for not living in the myriad of places where you do not reside? If asked why you chose your profession do you usually explain why you are not an opera singer, a Jain monk, a deep-sea diver or any of the thousands of professions you have not adopted?
It’s much simpler to focus on the positive reasons for using the determiners a/an or the.