he promised me that he would study harder this semesterthat 能省略吗,为什么

he promised me that he would study harder this semesterthat 能省略吗,为什么
he promised me that he would study harder this semester
that 能省略吗,为什么

he promised me that he would study harder this semesterthat 能省略吗,为什么
可以.但 若是在书面语中,一般不省.that是从句开始的“引导词”