English Grammar: a resource Book for Students

Imperative clauses as offers and invitations  410d
Friendly offers and invitations, which are commissive speech acts (→ 417–419), often 
occur in the imperative form:
A:  Have another drink.
B:  Oh, no thanks, I’ve had enough thanks.
A:  Come and see us some time if you’re in the area.
B:  Mm, yeah, that’d be nice. I will.
MODAL EXPRESSIONS AND SPEECH ACTS  411
When  modal  expressions  concerned  with  necessity,  obligation,  permission,  etc.  
(→ 377–407 ‘Modality’) are used in declarative and interrogative clauses, they often 
function as directives (realising speech acts such as commanding, instructing, suggest-
ing, advising, warning, requesting) and commissives (promising, offering, inviting).
The modal verbs can/could, will/would, shall/should, may/might, must, ought to 
are used frequently in this way:
[swimming instructor to learner]
You shouldn’t be looking at me Joseph, you should be looking out of the window.
(should is heard as a directive: the swimmer must do what the instructor says)