Modal verbs

Prohibition, permission, obligation

Have to and must express obligation. Have to is used for general obligation and must for orders and strong suggestions (from the person speaking).

• You have to be 16 to drive in this country.
• I have to wear a uniform in my job.
• I must remember to lock the door.
• You must stand behind the line.

Questions

• Must I pay for the coffee?
  NOT: Do I must pay for the coffee?

• Do I have to wear a uniform?
  NOT: Have I to wear a uniform?

To express no obligation we use don't have to.

• I don't have to get up early.
• We don't have to wear a uniform.

But must used in negation expresses prohibition.

• You mustn't smoke here.
  NOT: You don't have to smoke here.

• We mustn't arrive late.
  NOT: You don't have to arrive late.

The verbs need, allow and suppose, and the auxiliary verbs can and should can also be used to express prohibition, obligation and permission.