完成时态 be supposed toNancy is supposed to have finished the chemical experiment at least two weeks ago. 为什么句中要用完成时态,而不直接用be supposed to do?

完成时态 be supposed toNancy is supposed to have finished the chemical experiment at least two weeks ago. 为什么句中要用完成时态,而不直接用be supposed to do?
完成时态 be supposed to
Nancy is supposed to have finished the chemical experiment at least two weeks ago. 为什么句中要用完成时态,而不直接用be supposed to do?

完成时态 be supposed toNancy is supposed to have finished the chemical experiment at least two weeks ago. 为什么句中要用完成时态,而不直接用be supposed to do?
is supposed to have 就是用了be supposed to do的句型,have就是动词原形 看句子的意思呀~~表示Nancy应该在两个礼拜以前就完成了.而不是现在应该去完成. 所以要用过去完成时表达.

用完成时态是说明动作在某个时候己经完成，在这个句子里是指“化学试验应该在两周前己完成” 用不定式只能说开始做某动作，有没有完成不清楚，它与完成时态表达出来的意思是不同的，所以这个句子不能用动词不定式来替换。