Articles show whether a noun is definite or indefinite. That is, whether we already know which specific item(s) is/are being talked about.
Mistakes with articles usually don't stop the reader understanding your work, but they can make your point unclear, and many mistakes can make your writing look sloppy. Make sure you check your use of articles in your proofreading.
complete the sentences (NEF)
complete the text (Ox)
choose the right article: easier | harder (NEF)
spot the mistakes (Ox)