以下情況不可省略：
1.that引導主語從句、表語從句、同位語從句時不能省略。
2.賓語從句不止一個時,不能省略 如；they complain（that）they have to do homework from morning till night,that it is hard to study good,that they are very tired.是連續(xù)的賓語從句時,第一個可省,后面都不行。
3.賓語從句與主語之間有插入語時,不可省。如：the teacher said,happy and firmly(插入語),that we must do homework.
4.引導賓語從句是一個主從復合句。如：i belive that if i work harder,i will pass the exam.