if （it is） necessary是什么现象,假设都可以省略?

状语从句中有主语+be动词,后面跟动词被动态或形容词时主语和be动词同时省,比如你的那句.还有就是when he was asked about the murder,he said nothing.中he was也可以省,但是要同时省.什么现象我叫不出名字,但是是这种用法的.必须是状语从句中的从句才可以省略,而且要同时省..

扩展阅读：it is a challenge that ... if it isnt too much ... it is of necessity ... if it is up to me ... it is fun doing ... it is close by ... it is a beautiful day ... if ittoo much trouble ... it is a fun ...