Notice that the phrasal modals do not exhibit the same formal properties as the true modals in that the subject-verb agreement rule must be applied ( except for used to, which is an inflected past tense) and that all phrasal modals require that a to infinitive precede the main verb; that is, the phrasal forms behave syntactically much more like ordinary verbs than they do like true modals.
In fact, phrasal forms developed in part because the original class of modals lost their connection to time, and the phrasal forms gave English users a way to mark tense and express modality on one and the same verb form. Phrasal modals, however, differ from lexical verbs in speech in that they have assimilated with to and often pronounce to as if it were part of a single word with the verb: gotta, gonna, hasta, hafta and so on.
In many grammar texts be able to is presented as a substitute for can, However can is used at least ten times more often than be able to. Thereís one very important difference in meaning, particularly in the past tense
We could repair the old car
We were able to repair the old car
When couldis used it is potential that is implied . When were able to is used, the implication is that the actual event took place ( they actually repaired the old car)
The periphrastic modal be allowed to can sometimes be used instead of may particularly in those root modality uses interpreted as permission. This interpretation is clearly appropriate when be allowed to occurs with other simple modals
Will we be allowed to light the fire
When be allowed to is used in the past tense, the interpretation is remote fact, the permission was granted
We were allowed to have a break.
You can use have to or have got to instead of must to indicate that something is the case because of particular facts or circumstances. Thereís no past tense for must. Expressions of past necessity are generally presented via the form have to.
When I was at school we had to wear school uniform.
When have to is used in the present tense, it seems to provide an alternative to must particularly in expressing the root meaning of obligation. Have to can be used in a wider range of constructions than must
Do we really have to go to that meeting?
Yes, and we will have to present our report.
Thereís generally a preference for using have to when the obligation seems to come from some uncontrollable external source that compels an action
You can use going to instead of willfor expressing intention and prediction, however the form be going to is distinct in several ways. For example, be going to is not used to express the willingness associated with will. Typically, be going to has an implication that the future action is related to the present and will occur relatively soon after the time of speaking. If a distinction between immediate future ďnowĒ and more remote futureĽ laterĒ is being made then it will be done as in
Iím going to finish these exercises now
And Iíll get round to the others later
The immediacy conveyed by be going to as in Ö
Close your eyes . Iím going to give you a surprise.
Watch out! The monster is going to get you!
...may be connected to the literal source of the expression, with progressive aspect, suggesting that the subject is currently on a path moving to a goal. This sense of currently being on a path can also create an implication that that the action was already planned or decided.
In the following epistemic usesÖ
Iím going to be sick
I will be sick ( if I eat any more of this ice cream)
Öthe event is predicted to occur after the time of speaking . The difference between them contrasts the immediacy of going to with the type of conditional interpretation associated with will
The periphrastic modal be supposed to is used occasionally with a function similar to should in its root sense of weak obligation. Thereís an implication with be supposed to that the social requirement being mentioned is external to the speaker and may be one that the speaker feels is being ignored
Youíre supposed to be studying, not watching TV
Iím not supposed to be laughing about it, but itís very funny
The social obligations mentioned with be supposed to are generally treated as being weaker than those marked with should