这是一个省略句,
完整的句子应该是
Do you think that it is a good idea to have dinner together?（这是一个宾语从句的一般疑问句）
首先回答为什么用 to have 这里并不是什么语法的硬性规定,而是,
先看这句话
To have dinner together is a good idea,这是一个典型的主语+谓语+宾语的结构,
动词不定式To have dinner together 做主语,
is 谓语
a good idea 宾语,一起吃饭是个好主意.
在英语里面,如果主语太长,经常会用it当做形式主语来代替过长的主语,然后把过长的主语放到后面,就变成了 it is a good idea to have dinner together.
然后,问别人你觉得这是个好主意吗,前面加上Do you think that(宾语从句前面可以加that,也可以不加)
就变成了Do you think that it is a good idea to have dinner together?
再然后,为什么is可以省略?
变成Do you think it a good idea to have dinner together?
这是一个常用的省略,
之前的结构是个think后面的宾语从句,从句是it is a good idea to have dinner together
去掉is后就不是宾语从句了,而是think(动词)it(宾语)a good idea(宾补)的结构
明白了吧
类似的宾语从句省略成补语的例子还有
I found that my wallet was stolen 我发现我的钱包被偷了,
省略成
I found my wallet stolen
