With a modal verb:They 1have tied the knot if he had proposed to her.
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【正确答案】 1、正确答案：would    
【答案解析】解析：if he had proposed to her是对过去事实的假设，实际并未发生，根据时态一致原则，横线处应填表示过去“将……”含义的情态助动词。答案为would。