Must: Strong Obligation and Necessity

Must is a modal verb. This is great news for you because modal verbs are lazy. They do not change their shape. You do not need to worry about adding -s for he, she, or it. There is no -ing form and no -ed form. It is the same for everyone. It acts like a helper to the main verb. It sits right before the action to change the mood of the sentence. Think of it like a grammar traffic light. It tells the listener if the action is a green light (necessity) or a red light (prohibition).

1. 1Creating a sentence with must is a simple three-step process:
2. 2Start with your subject (I, you, we, they, he, she, it).
3. 3Add the word must or mustn't for the negative.
4. 4Add the base form of the verb (the dictionary form without to).
5. 5Example: You + must + wear + a helmet.
6. 6Negative Example: You + mustn't + smoke + here.
7. 7Notice that we never say must to wear. The to is a party crasher that isn't invited. Keep it simple and keep it clean. For questions, we usually swap must with the subject, but honestly, native speakers rarely do this anymore. We prefer using have to for questions because it sounds less like a Shakespeare play.

There are three main times to pull must out of your pocket:

• Official Rules and Laws: Think of airport security or road signs. You must show your passport. This isn't a suggestion; if you don't do it, you aren't getting on that plane.
• Strong Personal Necessity: This is when YOU decide something is important. I must call my mom. It is your internal clock telling you to act. Use this when you are making a promise to yourself.
• Logical Deduction: This is the "detective" use. If you see someone shivering, you might say, He must be cold. You are 95% sure it is true based on the evidence. It’s like connecting the dots in your head.
• Prohibition: Using mustn't tells someone that an action is forbidden. You mustn't park here. It is a big, red 'X' over the behavior.

Do not use must for the past tense. Musted is not a word, and it never will be. If you want to talk about a rule from yesterday, use had to. For example: I had to work late yesterday.

Also, avoid using must for polite suggestions with strangers. If you tell a waiter, You must bring me water, you might sound a bit like a movie villain. Instead, use could or can. Keep must for when the situation is serious or when you are talking to yourself.

• The "To" Trap: Many people say I must to go. This is the most common error. Remember: must + base verb. No to allowed!
• Confusing Mustn't and Don't Have To: This is a big one. You mustn't go means "Don't go! It's dangerous!" But You don't have to go means "Go if you want, but you don't need to." One is a stop sign; the other is an open door.
• Using it for Past Rules: As mentioned, must only lives in the present and future. For the past, had to is your best friend.

Must vs. Have to: This is the classic B1 showdown. Usually, must is about internal feelings or the speaker's authority. I must study means I want to pass. Have to is often about external rules. I have to study means my teacher gave me a huge test.

Must vs. Should: Should is a friendly piece of advice. You should try the cake is a nice suggestion. You must try the cake is a command from a very enthusiastic grandmother. One is optional; the other feels mandatory.