Here the modal auxiliaries can, could, may, might, should, will, would, and must are explained.
can: (physical or mental ability) Eg. Mary can swim very well.
could: (past tense of can) Eg. Ms. Hunter could not travel last year.
………..(possibility) Eg. For $80,000 you could buy a condominium. ………..(50% chance) Eg. It’s getting cloudy. It could rain later on.may and might usually have the same meaning. They usually express a 50% chance. Eg. It could rain later so you might need an umbrella and you may need your raincoat.
Might can suggest a slimmer possibility than may. Consider: “Mr. Urban may be at the office. His client might even be there too.”
Should is for expressing advice as in: “We could go out to the discos or we could go to a movie, but if we want to pass the exam, we really should stay at home and study.
With must there is a strong obligation as when an order or a command is given. “Drivers must obey the traffic lights.” Or, “You must eat in order to live.” With must the implication is that one has no choice.
Must is also used to express strong certainty as when you draw a logical conclusion. If when you look outside and see people carrying unbrellas you could conclude that it must be raining. If when you look outside you see that it is only cloudy but everything is dry everywhere and no one has an umbrella then you could say it can’t be raining.
Must not means prohibition. It is prohibited to park in front of a driveway so you must not park in front of a driveway.
Have to means must. But, do not have to means do not need to as in the case where something is not necessary. For example, “I don’t have to work on Sunday because it is a holiday.
To respond to the question: “Do you have to go to bed early tonight?” It is possible to reply, “Yes, I have to because I have an early appointment in the morning.” This would mean that I must because I have no choice. A negative answer could be: “I don’t have to because tomorrow is a holiday.” (meaning that there is no necessity)
After if were is used more frequently than was. When speaking formally were is considered to be more correct. The structure ….if I were you is said ofter for making suggestions as in, “If I were you, I would go to the dance.” When referring to situations that are impossible or just imaginary the past tense of the verb and would are used. Let me illustrate this with, “If I had a million dollars I would travel around the world.” This is said by someone who does not have a million dollars but who is dreaming about having it.
On the other hand, in “If Martha answers correctly, she will receive a prize,” we see how the present tense in an if clause and will in the second one, refer to the future. Here we are talking about something that is possible and can have real results. In other words, there is a true possibilty expressed in the if-clause. That possiblity is also a condition. If that condition is met, it will produce a real result. Martha will receive a prize (result) if she answers correctly (condition).
REFERENCE: How English Works; A Grammar Practice Book; by Michael Swan & Catherine Walter; Oxford University Press; 1997.