Should have done

[miladb]

When do we use the structure "should have done"? and particularly in conditional sentences?
for example is it correct to say: "If he were a hard-working student, he should have passed the exam"? (we know that he's not and didn't)

Thanks.

[philo2009]

Structurally possible, but unnatural, since 'should' in practice rarely occurs in counterfactual conditionals (we would expect 'would' here as a simple statement of hypothetical consequence).

On the other hand, a nonpredictive conditional such as

If he really is as conscientious as you say, then he should have passed the exam.

would be quite acceptable.

[philo2009]

Just for accuracy, I should add that an epistemic 'should' (of the type under discussion here, denoting an assumption) rarely if ever occurs in counterfactual conditionals. However a deontic (i.e. ethical) 'should' is quite possible, e.g.

If there were a fire, you should immediately leave the building.