Non-binding obligation: should, ought
Should and ought express a less strong obligation, which is not binding and may be unfulfilled. Referring to a past event, with should and ought + have + en, the speaker implies that the obligation was not fulfilled, as in 3:
1 People should drive more carefully.
2 You really ought to cut down on smoking.
3 I should have texted my mother.
These modals are used instead of must when the speaker lacks authority to impose the obligation. Tact, politeness or a lack of conviction of the absolute necessity of the predicated action are further motivations. The following invented advertisement clearly distinguishes the necessary from the merely desirable:
Candidates must be university graduates.
Candidates must be between 21 and 35.
Candidates should have knowledge of two foreign languages.
Candidates should have at least three years’ experience.
Functionally, these modal meanings are used to establish and maintain social relations and interaction. Through them speakers influence and control others, and commit themselves to certain courses of action. They may bring about changes in their surroundings by obligations which are met, permissions given, promises kept, and so on.
