Message:MUST 1. To express obligation or duty This also refers to laws and regulations. I must memorize all of these rules about modal verbs. People must remain seated until the show is over. You must wear a seatbelt at all times. 2. To emphasize the necessity of something Humans must have drinking water at least every two days. You must give up smoking, it's bad for you. We must have a special permit to camp in the national park. You must study the last two chapters before the test. Plants must have light and water to grow. You must drive carefully. 3. Deduction - Sure that something is true (Certainty) We use this when we don't know but we are certain that it is true (based on evidence). Look at all of that snow. It must be really cold outside. The ground was wet this morning. It must have rained last night. Dinosaurs were very big, they must have eaten a lot. It's five in the morning and you still haven't gone to bed? You must be tired! Jack must be home. I heard a noise coming from his room. 4. Expresses positive logical assumptions (Must + have + past participle) That must have been my mother calling me last night, nobody else has my number. He must have won the lottery with the new house and car he has just bought. She must have been at home - her car was there. 5. A strong recommendation Something that is highly recommended (stronger than using should) We really must get together for dinner sometime. You must see the new Peter Jackson movie, it's fantastic. The ice cream here is delicious. You must try some.