A descriptive grammar is a study of a language, its structure, and its rules as they are used in daily life by its speakers from all walks of life, including standard and nonstandard varieties.
A prescriptive grammar, on the other hand, specifies how a language and its grammar rules should be used.
Focus on delivery the message:
Who is your audience? 誰是我們的聽眾
Why are they listening to you? 為什麼要聽我們說話
What is the goal of the conversation? 這個對話的種點是
When do you want to see the result? 時限