The job of an article is to indicate the context of a noun (person/place/thing) as either general (by using ‘a’ or ‘an’) of specific (by using ‘the’).
When to use ‘a’ or ‘the’
We use the article ‘a’ when we are talking about something in general. For example, if you were talking or writing about ‘a’ textbook’, the listener/reader understands you mean any textbook in general.
We use the article ‘the’ when we are talking about a specific noun (person/ place/thing). For example, if you were talking or writing about ‘the textbook,’ the listener/reader knows exactly which textbook you are talking about.
When to use ‘an’