一般情况下是这样的。如：Jim likes science and he is an expert in it. 这个句子就不是主从句，而是并列分句，这时就不能省略be动词，就只能这样说：Jim likes science and is an expert in it. 祝你学习进步，更上一层楼！ (*^__^*)不明白的再问哟！请记得采纳，多谢！...
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...
