题目

doubt后什么时候用if 何时用whether?

提问时间：2020-12-12

答案

1.doubt如果是动词引导宾语从句时,如果主句是肯定句,则宾语从句的引导词可用if/whether.
如：I doubt if/whether he is a student.
当然,如果此宾语从句中含有or/or not时,只能使用whether.
如：I doubt whether he is a student or not.
I doubt whether he is a student or a teacher.
另：doubt如果是动词引导宾语从句时,如果主句是否定句/疑问句,则宾语从句的引导词只能用that.
如：I don't doubt that he is a student.
2.doubt如果是名词引导同位语从句时,如果主句是肯定句,则同位语从句的引导词只能用whether.
如：I have doubt whether he is a student.
doubt如果是名词引导同位语从句时,如果主句是否定句/疑问句,则同位语从句的引导词只能用that.
如：There is no doubt that he is a student.