省略的前提是要让省略之后的句子意思明白.当主语和谓语相同时,一般是可以省略的.如：
When he was studying in the classroom,he was trying hard to work out every problem.
=When studying in the classroom,he was...
一定要有主从句才能省略be动词和主语吗?
并列句主谓相同如果省略了,那可能就不是并列句了.
He is a atudent and he is in our school.
He is a student in our school.