We couldn't watch until our homework was finished.为什么用couldn't 换成cann't 行不行 could 和 can 有什么区别?

We couldn't watch until our homework was finished.为什么用couldn't 换成cann't 行不行 could 和 can 有什么区别? We couldn't see through the Heart breaen we couldn't cry togehter we couldn't live without others 'help we couldn't watch the bird show because of the heavy rain.（保持句意基本不变)the heavy rain____ _____ ____ _____ the bird show. We couldn't find them all.变同义句We couldn't find_____of_____. We shouldn't watch too much TV_____films. I couldn't find my lost watch if you didn't help me同义句 we never told him he couldn't do it翻译 we couldn't r__ enough money for a holiday We couldn't have played worse翻译啊 But we couldn't find them all为什么不是found We couldn't run in the classroom.(改为祈使句） we couldn't listen to each other clearly(肯定句) We couldnt walk around it on foot的意思 Walking past the park , I couldn't help__ some children flying kites.Astop to watch B stoping watch C stoping to watch D to stop to watch Because it rained,we couldn't play football的同义句_______ _______the_______,we couldn't play football If you want better grades,you ______ watch so much TV.答案是shouldn't ,可以用couldn't吗shouldn't ,couldn't ,有什么区别呢