2．表示委婉或客气
  虚拟语气（could, would, might +动词原形）可使说话者的口气变得委婉客气。    
  You could answer this email for me.你可以替我回这个电子邮件。
  Could you leave me your telephone number and address你能将电话号码和地址留给我吗？
  Would you mind opening the window劳驾把窗子打开，好吗？
  You might as well put off the discussion till next week.你们不妨把讨论推迟到下个星期。

  比较：
  would lide to do
  愿意，想要
  would lide to have done
  本来想
  I would like (to have) a word with you.我想和你谈一谈。(现在想)
  I would like to have talked with you.我原本想和你谈一谈的。（没有谈成）

  3．表示惋惜或责备
  Given more time, we could have done better.如果给我们更多时间，我们能够干得更好些。（我们并没有得到更多的时间）
  You could have got up a little earlier！你完全可以早点儿起来！（实际上没有早起）
  It was cold yesterday. I should have worn a heavy coat.昨天很冷，我该穿件厚外套的。（但我没穿）
  This wall shouldn't have been pained blue.这墙不应该漆成蓝色。（但已漆了）
  提示：
  当代英语一个显著的变化就是虚拟语气的使用越来越少，许多该用虚拟语气的地方都用陈述语气代替。
  He suggested that I went to the hospital at once.他建议我们立刻去医院。（原应用should go）
  We shall write down the address lest we forget.我们要把地址记下来，以免忘记。（原应用should forget）
  Imagine you are an astronaut.设想你是个宇航员。（原应用were）

情态动词