Grammar Reference

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).

Questions

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

But must used in negation expresses prohibition.

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