状语从句省略不是涉及该用动词的ed形式还是ing形式或have +动词ed.这样的用法如何判断,高手帮忙.0分
答：状语从句省略"主+Be"条件是其主语与主句的相同或为it谓语中有be动词如When (water is) heated, water will become vapour.
When (you are )crossing the street, you should be careful.
If (it is) necessary, I will help you.
没有第三情况.