he promised me that he would study harder this semesterthat 可以省略吗,

he promised me that he would study harder this semesterthat 可以省略吗,
he promised me that he would study harder this semester
that 可以省略吗,

he promised me that he would study harder this semesterthat 可以省略吗,
宾语从句中that不做成分可以省

可以，THAT在句中不做成分

可以 宾语从句中that可以省