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·        The meaning of must is "The way I see/feel it in the present circumstances, it is necessary to.." (The speaker feels/admits/endorses the idea of obligation).

·         Must expresses obligation of many different natures

  1. Moral obligation: You must pray and go to church.
  2. Obligation as strong advice: You must stop smoking, James.
  3. Obligation in view of circumstances (obligation as necessity). We must slow down, Tom: the police are behind us.
  4. Obligation by law, from authority: Minors must not be served alcoholic beverages.
  5. Logical obligation (deduction): You must be the man who phoned yesterday night.


·        Have to expresses the same range of obligations when they are not felt personally.

Their negative forms

Mustn't still expresses an obligation: a prohibition is after all, a negative obligation, an obligation not to do something.

You mustn't take sweets from strangers

Don't have to expresses a lack of obligation: it's equivalent to Don't need to or Needn't. There is no obligation.

You don't have to brush your hair every ten minutes, darling.

Perfect modals of obligation

                   Can you express the idea of present obligation for a past fact? How can you feel now that something is compulsory, if it has happened already? It is impossible. . The only kind of obligation that can be felt now about something past is logical obligation (deduction) (the opposite is can't + perfect modal)


 You must have been very rich. è Talking about the past, but the obligation is felt now.

You had to be rich( if you wanted to mix with them).è  The idea of obligation is past, as well as the circumstances.

You had to pray every morning in that school. è Again, a past situation and a past obligation.






  • The modals of possibility (may, might, can, could) mean something like: "The way I see/feel things in the present circumstances, it is possible that.." For may and might, we should add: (..if I have anything to do with it).
  • Could and might express remoteness of any kind (past time, unlikely occurrence, stiffness -due to shyness or respect-.. )
  • May and might express a more personal involvement in the creation of the possibility (an extrictly personal opinion, or some kind of authority..)

There are many kinds of possibility:

  1. Probability, chances: This could be your lucky day (It is likely, possible that..)
  2. Theoretical possibility, options: We can be friends, if you want to.
  3. Ability: He can dance on his nose (He is able to..)
  4. Assumptions (logical possibility): This may well be the purse you lost
  5. Permission: Can I go to the cinema? –No, you may not. (You’re not allowed to..)

Their negative forms

We use can’t to express that something is logically impossible

He can’t be Julio Iglesias, with that awful watch!

The meaning is kept in the perfect form:

She can’t have broken your washing machine. She wasn’t here yesterday. (Talking about past facts, we consider them impossible now)

Compare: She couldn’t break your washing machine. (Remote action in time: maybe she tried to break it but was not able to.) But we are not looking back from NOW. It is just a past action.

Perfect modals of possibility

One of the most common uses of perfect modals of possibility is for assumptions about the past: we look back on an action that happened before, but we consider it now. That is how we can discuss the possibility of past facts now, or make regrets:

It could have been a great opportunity. You could have been a bit more talkative!

Compare: It could be a terrific day for Man U.. (Now we think there is a remote chance that it will be a terrific day) It could have been a terrific day... (We now think that it was possible before that it was a terrific day . We assume that it is impossible now).

-He might have been trapped in the mud. (We now think so)

-He might be trapped in the mud. (action now, possibility now as well)

-They might be the winners. (present winners)

-They might have been the winners. (we think so now, but this sentence sometimes suggests they weren’t).


POSSIBILITY AND OBLIGATION Overhead Projector Transparency

 Questions to ponder -no answer provided-

What is the nature of this possibility?

Carmen cannot replace me.

What is the past of CAN?

I cannot come today, but I could come tomorrow.

Who may say the following sentences?

May I suggest the pudding?

Might I suggest the ostrich drumsticks?

Typical conversation between child and parent: (explain the reply)

-Can I cross the puddle with my wellies and splash the car?

-Oh dear: you are so cute! No, you can, but you may not.

Can you see any difference between..

This could be your lucky day.

This might be your lucky day.

What's the difference between..

You may not visit your boyfriend until he gets a proper job.

You may find your glasses today. Don't lose heart.

Nature of the obligation in:

You must be very patient with the children. Otherwise, you won't cope.

You must be very patient with the children: they seem to adore you.

You must be very patient with the children: their parents are watching you.

You must be very patient with the children to get the job at the local nursery school.

You must be very patient with the children. They don't mean to be a nuisance.

Apparently, Tommy caught the train yesterday. But, what kind of obligation is conveyed? When is it felt?

He had to catch the early train to Glasgow.

He must have caught the early train to Glasgow.

Couple the sentences according to similarity in meaning (there is an odd one out):

Rose will have to read the instructions leaflet again.

Rose has to read the instructions leaflet again.

Rose can read the instructions leaflet again.

Rose must read the instructions leaflet again.

Rose should read the instructions leaflet again.


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