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.
Expository writing differs from other forms of writing, such as fiction and poetry. In fact, this lesson itself is an example of expository writing. The expository essay is a tool that is often used in the academic world. If you've attended school, it's highly likely you've written one.