How to write a commentary?

First and most important point about writing commentary: make sure you read it! Read the passage at least twice, the first time to get a general gist of what the passage is about, the second time make mental notes of interesting features for discussion. This goes for both seen and unseen passages - even if you know the passage. Remember that what a commentary does to comment on the passage, not describe what happens. So avoid describing "what happens", go straight to "what is the significance of what happened", "how does the author achieve the intended effects using literary devices", "what is the impact on the reader", and "what is the passage's relation to the context". Keep in mind these questions when writing about the passage, always refer to the text itself using brief quotations (no more than three words will do) and line numbers when comparing different parts of the text (e.g. the repetition of "again" in lines 4 and 6). Finally, enjoy the text!