高一英语暑假作业本P7 __he will attend the meeting is certain

That he will attend the meeting is certain这是一个由that引导的定语从句,还可以写为It is certain that he will attend the meeting.
翻译：他将去参加/出席这次会议这件事是一定的.那请问为什么不用whether呢因为句子的意思是“是一定的‘，所以不存在”是否“。