三种基本形式 If had a bike.I would lend it to you(f从句用过去时,主句用 should/ would/ could/ mighti不定式) If he had been here he would have helped you(从句过去完成时,主句 should would/ could/ might+不定式完成时) I would do it some other wa(从句用 should/ were to3主句用 should/woud could /might+不定式)