过去 Had done Would* have done
现在 Were/did Would* do
将来 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.[过去]
If he smoked less,his cough might be well soon.[现在]
If the lecturer should be late,you would have to make a speech first.[将来]
主动:We must keep this in mind.
被动：This must be kept in mind.
我们必须把这个记在心里.