I don't know 主句的“我不知道” 就使从句附上了不确定的（uncertain) 的意思, 故用if/ whether

I don't know if he he will go to japan next week.

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祝你学习进步,更上一层楼!(*^__^*)】那he is a student也用i don't know,变宾语从句，为什么他就是＋that?如果用 i don't know that he is a student. "我不知道他是个学生（这个事实）” 我不太认同这个说法。因为，如果你说话的时候已经知道他是个学生了，要说不知道，那是过去的事i didn't know he was a student.再如：我不知道你在这。I didn't know you were here. (说话的时候应该知道对方在这了）