在英语语法中，名词性从句的从句中，什么时候该用should+动词原型呢，我看很多名词从句句子都有，语法书

表示 一个坚持insist。二个命令order. /command。三个建议advise/advice/suggest/suggestion/propose/proposal.四个要求(require/request/demand/urge)的名词后如果有同位语从句。或是动词后宾语从句。表语从句这些名词性从句等。都 用should +动词原型。should可省略。 如
His suggestion is that we should start now .表语从句是根据前面的suggestion的意思而用了虚拟。
He suggested that we should start now.宾语从句也是由于suggestion这个词用了虚拟。
His suggestion that we should start now is reasonable.同上。

楼上说的这些词的名词形式
如 My sugestion is that we should take an umbrella with us for fear that it should rain.

command，order，suggest，request，require等有关命令建议请求的词都加should do。 还有虚拟语气等等