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.