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COULD HAVE + PAST PARTICIPLE | 
      
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Look at the following sentences expressing an action 
        and an alternative: | 
      
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ACTION: 
        She came at 10 o'clock. | 
      
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ALTERNATIVE: 
        She could have come earlier (but she didn't). | 
      
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In both examples above the speaker is expressing an 
        opinion about an action that occurred in the past and saying that there 
        were alternative courses of action available at that time but which 
        were not adopted. | 
      
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SHOULD HAVE + PAST PARTICIPLE | 
      
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This construction is used in two principal ways: | 
      
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a) In the negative to show that, in the speaker's 
        opinion, a particular past action was a mistake or was regrettable: | 
      
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-  He
        shouldn't have taken so long to finish his report (but he did it). | 
      
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b) In the affirmative to indicate that there was a 
        better alternative to what was done: | 
      
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-  They
        should have reduced their prices sooner (but they didn't). | 
      
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