注:1,but for更不是
2,if only 要是...就好了(时态倒退一格)
对过去 if only+主+had done
对现在 if only+主+did/done
对将来 if only+主+would(could,minght)+do
3,only if,=if
时态倒退一格
从句谓语 主句谓语
与现在相反 did/were would/should/could/minght do
与过去相反 had done .../.../.../... have done
与将来相反 did/were to do/should do .../.../.../... have done