Must and Must Not

Must 1. Must do, have to do, and need to do can have the same meaning.  

They can all show necessity, obligation, or duty.  

必要性, 义务, 责任, necesidad, obligación, deber, необходимости, обязательства, обязанности, ضرورة والتزام وواجب

Must                                                                 Same Meaning                                           

He must take his medicine every day.                He has to/needs to take his medicine every day.

I must change the baby's diapers. (尿布, pañales)                             I have to/need to change the baby's diapers.

Parents must teach their children good manners.                                 Parents have to/need to teach their children good manners.

In everyday spoken English, we use have to and need to more often than must.

In written English, we usually use must (not have to or need to) when the instructions are official or from an authority.

official: 官方的, 法定的, 强制的, oficial, autorizado, официа́льный, رسمي،

authority: 当局, 官方, autoridad, власть, власти, سلطة

All visitors must check in at the front gate. (a sign at the entrance to an army base*) *军 事 基 地, база, قاعدة   

All passengers must carry a valid pass or transfer at all times. (a sign on a Muni bus)

When must means have to or need to, the negative is don’t have to (not must not!).

Present Affirmative        Same Meaning                                         Present Negative            

I must go.                     I have to go. I need to go.                      I don’t have to go.

She must work.             She has to work. She needs to work.      She doesn’t have to work.

Past Affirmative                                                                     Same Meaning                              Past Negative          

I had to go.                   I needed to go.                                       I didn’t have to go.

She had to work.           She needed to work.                               She didn’t have to work.

Must 2. Must not is different from don’t have to and don't need to!

Must not means that something is *forbidden, prohibited, or a very bad idea. Usually, if you do it, something bad will happen.

* 禁止, 沒允許, no debe, no se puede, es prohibido, запрещено, запрещено, ممنوع، ويحظر.

You must not light a match when you are filling up your tank at the gas station.

You must not walk on the BART tracks. (轨 道, vía, путь, مسار) They carry live electricity.

You must not let a young child under 60 lbs. ride in a car without a child seat. It’s against the law.