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Unit 5
G rammar: Modal verbs
Exercises: 1,2,3,4
Text: Advantages of social co-operation
Oral topic: Sports
Modal verbs
May/might (not)
These refer to a present or future possibility. May is usually slightly more certain than might.
In England, unexpected guests may/might be offered some tea. In future, people may/might not need to travel to business meetings.
May/might (not) have
The modal perfects refer to a past possibility.
Life when my parents were young may/might have been less demanding than it is now.
Must and can't
Look at the two clauses in this sentence:
Jackie has travelled a great deal, so she must know a lot about
different countries.
In the first clause the speaker means 'it is a fact that Jackie has travelled a great deal'.
In the second clause the speaker means 'I am sure Jackie knows a lot about different countries because I have worked it out from the evidence (the fact that she has travelled a great deal).' Can't is used to show that the speaker has considered the evidence and is sure that something isn't true:
Jackie has only spent a few days in Canada, so she can't know much about the country.
Must have done and can't have done
These are used to come to similar conclusions about the past: Jackie must have been to South Africa because she's got a lot of photographs of Cape Town.
Jackie's told me about every country that she's visited, and she's never mentioned Egypt, so she can't have been there.

Should, had better, ought to

These are used to give advice, or say what we think would be a good thing to do now or in the future.

• 
  Should and ought to mean the same, but should is used much more often.
  

The team should spend / ought to spend less time chatting, and more time training.

• 
  Shouldn't is used to give advice about what not to do. Oughtn't to is also possible, but is used less and less.
  

The team shouldn't spend so much time chatting.

• 
  Had better (usually shortened to'd better) normally refers to the present situation, rather than the future, and is more informal than the other two. It is used in speech more than in writing. We'd better do some more practice before the match.
  

Should have done and ought to have done
These refer to the past, and are often used for criticism because an action didn't happen.
We should have spent / ought to have spent longer practicing.
Shouldn't have done and (occasionally) oughtn't to have done
These refer to the past, and are often used for criticism because an action happened.
We shouldn't have spent so long chatting. (We've missed our train.)