Key considerations
Every time we use a noun we have to decide whether or not to use an article, and if we decide that an article is necessary, we then have to decide which one. We base these choices on a complex interaction of factors including meaning, shared knowledge, context and whether the noun is singular, plural or uncountable.
In many cases, however, fixed expressions and idioms require us to use a particular (or not to use an article at all), apparently contradicting these basic rules. Knowing these expressions is a significant factor in using articles correctly.
In helping learners to understand and use articles (particularly if their first language is a non-European language and does not have a broadly equivalent article system), we need to focus their attention constantly on how articles are used in texts they read, beginning with. the most accessible and generalisable principles. There is little point in correcting mistakes and giving learners practice exercises and activities until they have developed a good awareness of how we use articles.