该句的意思是“是否”相当与"if"，用来表达两可的情况。也可以说"whether or no"。
1. I'll see whether or not I can persuade him to accept it.
2. He seriously debated whether or not he should charge his tenants extra for the water.
3. He could not decide whether or not to return home.
A: I don't know whether or not to believe him.
B: You can trust him. He always tells the truth.
A: But it's really too important to me.
B: I said you can trust him. You just have to believe it.