Modals: must / should — Explained

Modals are special verbs that express ideas like obligation, advice, possibility, or permission. Must and should are two important modals that English learners often confuse because they both express some kind of obligation or recommendation, but they have different strengths and uses.

Must: Obligation and Necessity

• Use must when you need to express a strong obligation or necessity. Something is required or very important to do. Must shows that something is not optional.
• You must arrive on time. (This is required.)
• Students must complete their homework before class. (This is a rule.)
• You must see a doctor if you have severe pain. (This is necessary for your health.)
• We must protect the environment. (This is essential.)
• Use must also to express a logical conclusion or what you think is certainly true.
• She has not eaten all day. She must be very hungry. (You are sure about this.)
• That noise is so loud. Someone must be using power tools. (You believe this is true.)