should do 表示应该做, 一般指现在或将来 的动作, 指过去的动作时应站在过去的角度。 should have done 表示本应该做而没有做的。 站在现在的角度上评说过去的事情。 You should have written to your mother. My teacher said I should study harder. I should have studied harder when I was young.