I'll paste from the New Oxford, but I can't really sum it up any shorter or more understandably myself. Basically, there are some kind of weird and antiquated rules, and they are largely disregarded modernly. should modal verb (3RD SING.
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Qingqing1.I'm surprised that he ____ to help me when I needed his help .
A. should refuse B . would have refused C. should have refused D . may have refused
2.Isn't it strange that the lazy boy _____ pass the exam!
A.should B.has C.might D.would
Key: 1.C 2.A
How to understand the time between