Definition and explaination
When do you use/do it?
You tend to use commentary when you are writing an essay or you are given a form of writing and need to explain further its connection. For example you can use commentary to connect literary devices to the purpose of a given story to enhance your learning and interpretation of the article.
How do you use/do it?
You write commentary when you want to further show your teacher (or yourself) the connection you had with certain aspects of literature through writing. For example, you would write commentary if you had to connect satirical features of a movie to the purpose of the movie.