如题所述
if 引导虚拟句:
if 从句假设‘过去’情况用had done表虚拟,主句如果表过去用would/could/should have done 表虚拟,主句如果表现在或将来用would/could/should do 表虚拟
if从句假设一般时情况 用were/过去式
if从句假设将来时情况 用were/were to do/should do