为什么用shall的过去式?You should do it at once

they should do it.at once They should do it at once. 英语翻译we should do it at once (该为同意句)we - - - do it at once They should do it at once.改成被动句. you shoud do your homework at once转为同义句 you should do your h The work should _______(not do)at once.用动词的正确形式填空 you should do it .这里的should是情态动词吗?它的过去式和原型分别是什么么啊? shall和should的过去式是什么? should是shall的过去式吗? The suggestion____you should do it at once is not the one __ You should tell your mother about it at once.(同意句） =You __ _ You should a doctor at once