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 17 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.

Prohibition	Obligation	No obligation	Permission
not supposed to go	supposed to go	--	--
shouldn't go	should go	--	--
--	need to go	don't need to go	--
--	have to go	don't have to go	--
can't go	--	--	can go
mustn't	must	--	--
not allowed to	--	--	allowed to