My suggestion that we should stick to our original plan was finally accepted.

这里that we should stick to our original plan 是同位语从句.看了下面的语法解释和例句你就明白了.
1.在复合句中用作同位语的从句叫同位语从句.它一般跟在某些名词后面,用以说明该名词表示的具体内容.
　　I heard the news that our team had won．我听到了我们队获胜的消息.
同位语从句
2.可以跟同位语从句的名词通常有news,idea,fact,promise,question,doubt,thought,hope,message,suggestion,words（消息）,possibility等（一般的“抽象”名词都可用）.
　　I’ve come from Mr wang with a message that he won’t be able to see you this afternoon． 我从王先生那里来,他让我告诉你他今天下午不能来看你了.
3.英语中引导同位语从句的词通常有连词that,whether,连接代词what,who.连接副词how,when,where等.（注：if,which 不能引导同位语从句.）
　　He must answer the question whether he agrees to it or not.他必须回答他是否同意这样一个问题.
4.有时同位语从句可以不紧跟在说明的名词后面,而被别的词隔开.
　　The thought came to him that maybe the enemy had fled the city． 他突然想起可能敌人已经逃出城了.