Do you think it necessary for me to give him a letter 这句话为什么

1,it在整句中作宾语,（you是主语,think是谓语）.
2,necessary 是宾补,补充it
3,it其实是形式上的宾语（在整句形式上看是宾语（,实际宾语是：to give him a letter 这个不定式.it是代替这个不定式的（为避免头重脚轻,就是说宾语太长了）.
4,如果用it is necessary ,那又是主谓宾了,就是句子了,也就是宾语从句.这里think后面得宾语从句要加that引导.
Do you think that it is necessary for me to give him a letter .也是可以的.

请教一下上面的朋友。如果necessary是宾语补足语，那么to give him a letter是什么成分？ 谢谢

Do you think it necessary for me to give him a letter 这句中的necessary是宾语补足语，补充说明宾语it
Do you think it is necessary for me to give him a letter 。
此句加上it完全可以，不过it is necessary for me to give him a letter 就成了宾语从句

think it +adj+for sb +to do sth
eg:
I think it is haapy to meet you here today.
Do you think it necessary for me to give him a letter 这句中的necessary是宾语补足语，补充说明宾语it
而Do you think it is ...