一、若条件句中有were,had,should,可把if省略,而把were,had,should放在主语前,形成倒装结构。否定句中not放在主语后。Had you not come late/Had you come earlier,you would have caught the bus.二、but for(要不是),without(假若没有),otherwise(否则)等含蓄条件虚拟,句子的时态要依据具体情况而定。Without electricity(=If there were no electricity),life would be quite different today.(从today可知主句是对现在的虚拟,用would do的形式) First, if the conditional sentence were were, had, should, can be omitted, and had, had, should be placed in front of the subject, the formation of flip-chip structure. Negative sentence not on the subject. Had you not come late / Had you come earlier, you would have caught the bus. 二, but for for, without, otherwise If you implicit, the tense of the sentence depends on the circumstances And set. Without electricity (= If there were no electricity), life would be quite different today. (From today we can see that the main sentence is virtual for now, in the form of would do)