Father promised that if I studied harder he would take me to Beijing next summer.that不能省略吗?可是,那道题只选that if,不选if.Why?

Father promised that if I studied harder he would take me to Beijing next summer.that不能省略吗?可是,那道题只选that if,不选if.Why?
Father promised that if I studied harder he would take me to Beijing next summer.
that不能省略吗?
可是,那道题只选that if,不选if.Why?

Father promised that if I studied harder he would take me to Beijing next summer.that不能省略吗?可是,那道题只选that if,不选if.Why?
不能省略
that之后跟了一句句子
那句句子里面还有条件从句
if是属于包在里面的那个从句的