PersidentLincoln,walkingwithafriend,turnedbacktoassistabettlethatlayonitsbacks,legsclawingtheair,vainlytryingtoturnitselfover.legs不会是名词吗?为什么后面跟动名词clawing?Geneticengineeringhasalreadymadevastp

Question

Persident Lincoln,walking with a friend,turned back to assist a bettle that lay on its backs,legs clawing the air,vainly trying to turn itself  over.legs不会是名词吗?为什么后面跟动名词clawing?
Genetic engineering has already made vast progress  even though Crick and Watson had discovered DNA's "double helix"conformation only a relatively short time ago.这句话为什么要把had discoverded 改成discovered?
In any Puerto Rican home,even the poorest tradition demanded that whatever food be avaliable be shared.为什么要把 whatever food be avaliable 改成whatever food was avaliable

Answer 1

clawing the air 现在分词作后置定语修饰legs

首句用的是现在完成时强调当前产生的影响,与之相对的过去事实只能用一般过去式不能用过去完成时.
在任一个波多黎各人的家中,最基础的礼节是要求（家中）所有可以用的食材都要（与客人）分享.be available肯定不对,从句时态要与主句时态一致,由demanded得知使用过去式故改为was.