1must 
/ˈmʌst/ verb
/ˈmʌst/ verb [modal verb] 1 a somewhat formal in US English — used to say that something is required or necessary ▪ You must stop. [=you have to stop; I command you to stop] ▪ I told him what he must do. [=what he had to do] ▪ One must eat to live. ▪ You must follow the rules. ▪ We must [=have to, need to] correct these problems soon or the project will fail. ▪ I must remember to stop at the store. ▪ “Must you go?” [=do you have to go?] “Yes, I'm afraid I really must.” ▪ “Must you go?” “No, I don't have to.” ▪ If you must go, at least wait until the storm is over. ▪ It must be noted, however, that the company was already in financial difficulties. ▪ We must keep/bear in mind that she didn't have any previous experience. = It must be borne in mind that she didn't have any previous experience. ◊Note the difference in meaning between must not and not have to.
▪ You must not do it. [=it is necessary that you do not do it; I command you not to do it] ▪ You don't have to do it. [=it is not necessary for you to do it]
b — used to say that something is required by a rule or law ▪ All passengers must exit at the next stop. ▪ Students must have completed Sociology 101 before they can take Sociology 102. 2 somewhat formal in US English — used to say that someone should do something ▪ You must [=ought to, should, have to] read this book. It's fantastic! ▪ You must come visit us soon. [=we would like to have you come visit us soon] ▪ You really must see the doctor about that cough. [=I urge you to see the doctor; I really think you need to see the doctor] 3 — used to say that something is very likely ▪ It must be almost dinner time. ▪ She must think I'm a fool. ▪ It must have been the coffee that kept me awake. ▪ He must have been the most gifted student at the school. [=I think he was the most gifted student] ▪ He must have a lot of money to live the way he does. ▪ The bus must be coming soon. ▪ You haven't eaten all day. You must be hungry. ▪ You're going to wear that? You must be joking! ▪ You must have been very worried. ▪ You must be thrilled about the new baby. ▪ If he really was there, I must have seen him, but I don't remember seeing him. ▪ There must be some mistake. 4 — used in various phrases to emphasize a statement ▪ I must say, I was surprised to hear from him. ▪ She's a talented actress, I must say. ▪ She's a talented actress, you must admit. ▪ I must admit, I expected better results. ▪ I must warn you, this will not be easy. ▪ I must confess, I haven't actually read the book yet. 5 somewhat formal in US English — used in questions that express annoyance or anger ▪ Must you be so unreasonable? [=do you have to be so unreasonable?] ▪ Why must it always rain on the weekend? [=why does it always have to rain on the weekend?]
▪ You must not do it. [=it is necessary that you do not do it; I command you not to do it] ▪ You don't have to do it. [=it is not necessary for you to do it]
b — used to say that something is required by a rule or law ▪ All passengers must exit at the next stop. ▪ Students must have completed Sociology 101 before they can take Sociology 102.
if you must
— used to say that you will allow someone to do something even though you do not approve of it ▪ You can smoke if you must, but please do it outdoors.
if you must know — see 1know
