引导词 “if” 既可以表示 “如果”，也可以表示 “是否”。在表示 “是否” 的意思时，有时 “if” 和 “whether” 可以互换使用。这就难倒了一些英语学习者 —— 什么时候该用 “if”？什么时候该用 “whether”？
 
When do we use if and when do we use whether? Let's take a look.
The simple rule is that with conditional sentences we use if, and when we have two options, we use whether.
If I get hungry, I'll have a snack.
But I don't know whether to have an apple or a biscuit.
When asking an indirect yes/no question, we can use either if or whether – both are correct.
Do you know if the apples are fresh?
Do you know whether the biscuits are any good?
Whether is maybe a little bit more formal, but with both questions there are only two possible answers – yes or no.
 
用法总结
条件句中，用 “if” 表示 “如果”, 不能用 “whether”。
If I get hungry, I'll have a snack.
I'll take an umbrella if it rains.
陈述句中提及两种选择时，常用 “whether”。
I don't know whether to have an apple or a biscuit.
I can't decide whether to go home or not.
间接一般疑问句中，“if” 和 “whether” 都适用，同样表示 “是否” 的意思。“Whether” 更正式一点。
Do you know if the apples are fresh?
Do you know whether the biscuits are any good?
Can you tell if it's going to rain today?
Can you tell whether it's going to rain today?