以下情况不可省略：
1.that引导主语从句、表语从句、同位语从句时不能省略。
2.宾语从句不止一个时,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.是连续的宾语从句时,第一个可省,后面都不行。
3.宾语从句与主语之间有插入语时,不可省。如：the teacher said,happy and firmly(插入语),that we must do homework.
4.引导宾语从句是一个主从复合句。如：i belive that if i work harder,i will pass the exam.