第1个回答 2012-12-27
If it rained tomorrow,our picnic would be put off
=If it should rain tomorrow,our picnic would be put off
=If it were to rain tomorrow,our picnic would be put off
这三个意思相同,都表示对将来情况的虚拟
要注意的是
If it rained now,our picnic would be put off
rained还可以表示对现在的虚拟
所以did有两种可能性,
should do, were to do 只有一种可能性!