Finish some exercises which will help you understand the concept better.
In this video tutorial you'll learn about the imperative and how we use it. You'll also learn when we use the modal verbs Must and Mustn't. There are several examples through out the video followed by Practices, which you'll have to do.
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Watch the video titled "Imperative, Must and Mustn't".
Pause during the video (at the Practice parts) and do the exercises I have assigned for you in the Google Form bellow the video. Each section is clearly marked "Practice 1", Practice 2"...