MODAL OBLIGATION
Inescapable obligation and necessity: must, have to, have got to, gotta, shall
In English, deontic obligation and necessity can be thought of as an inescapable duty or requirement, realized by must, have to, have got to, and in a lesser degree by shall; or else simply as an advisable course of action, realized by should and ought. Must can have the force of a command.
Must as a modal of obligation
When realized by must and with the addressee as subject, obligation can have the force of a direct command, as in 1, although modal lexical verbs are more explicit. Compare You must go with I urge you to go, I order you to go. Order with a 1st person subject is too strong for ordinary use.
1 You must try harder.
2 We must do better than this. (MP after an election result, sharing the responsibility with others)
This force derives from the fact that (a) in certain cultural contexts such as school, family, the Armed Forces, the speaker has authority over the addressee, who is the subject ‘you’; (b) the speaker takes the responsibility for the action being carried out; and (c) the verb is agentive and in active voice.
The force of must is diminished if one of these factors is modified, providing useful strategies to mitigate the directness of the obligation, although not its inescapability. Such is the case in 3.
3 Crimes of violence must be punished. (non-human subject, authority doesn’t reside in the speaker, passive voice)
4 Applications must be in by May 1st. (non-agentive verb; passive, 3rd person subject)
When no human control is implied, the meaning is that of deontic necessity.
5 Lizards must hibernate if they are to survive the winter. (it is necessarily the case that)
Shall, have to, have got to, gotta as modals of obligation
Of all the modals of obligation, shall is the most imperious, direct and subjective, and for this reason is little used in the spoken language. It occurs in legal and other formal contexts such as the regulations of the Olympic Games.
Of the lexical modals, have to is objective (the obligation is external), and have got to subjective (the obligation is internal). Compare 2 and 3:
Syntactically, have to, unlike must and have got to, has non-finite forms, having to and have to. Both have to and have got to have a past form had (got) to. Only have to can combine with the modal auxiliaries (* may have got to /may have to), as in:
I may have to go to Washington for a few days.
*I may have got to go to Washington for a few days.
Must has no past form as it is, historically, itself a past form. Forms of have to are therefore brought in to express past and future obligation.
1 All competitors in the Games shall wear a number.
2 I’ve got to go now. (contracted colloquially to I gotta go now, especially in AmE) (obligation internal)
3 I have to go and see the Dean. (obligation external, both AmE and BrE)
4 It has to be unacceptable. (Foreign Secretary William Hague referring to chemical warfare)
5 We had to pay in advance. We’ll have to pay in advance. (external)
