Instead of submitting a copy of your teaching certificate with your application, you may submit verification that you are matriculated in a certification program (e.g., letter of acceptance, letter from advisor).
Expository writing is writing that seeks to explain, illuminate or 'expose' (which is where the word 'expository' comes from). This type of writing can include essays, newspaper and magazine articles, instruction manuals, textbooks, encyclopedia articles and other forms of writing, so long as they seek to explain.
Try clustering. Write a brief explanation of the subject of your expository essay on the center of a piece of paper and circle it. Then draw three or more lines extending from the circle. Write a corresponding idea at the end of each of these lines.