過去 Had done Would* have done
現(xiàn)在 Were/did Would* do
將來 Should do/were/were to do Would* do
例句：If there hadn’t been much rain in spring,we would have had a good harvest now.[過去]
If he smoked less,his cough might be well soon.[現(xiàn)在]
If the lecturer should be late,you would have to make a speech first.[將來]
主動(dòng):We must keep this in mind.
被動(dòng)：This must be kept in mind.
我們必須把這個(gè)記在心里.