Learners with certain native languages leave out the definite and indefinite articles when they write in English. Why is this?
Well, it’s rather simple, their languages don’t have articles, so it is tricky for them to remember to include them in English. However, this problem, unlike other grammar mistakes, is easy to fix.
Generally speaking, in order for an intermediate and advanced learner of a language to correct a grammar mistake, they need to be aware that they make the mistake and then they need to deliberately practice to correct the error. In the case mentioned above, i.e. learners forgetting to include articles, I advise them to read through every essays after they finish writing and find all the nouns in the text and then to make sure they have included the appropriate article (unless of course it is not necessary, e.g. uncountable nouns).
If you make grammar mistakes and want to correct them, it is important not to focus on more than one grammatical error at a time because this will drastically increase your chances to actually correct the mistake. Usually, after several weeks of deliberate practice, you will be able to avoid the mistake you were struggling with.