VOA一分鐘英語：如何區(qū)分引導詞 if 和 whether？

引導詞 “if” 既可以表示 “如果”，也可以表示 “是否”。在表示 “是否” 的意思時，有時 “if” 和 “whether” 可以互換使用。這就難倒了一些英語學習者 —— 什么時候該用 “if”？什么時候該用 “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.
 
用法總結
條件句中，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.
陳述句中提及兩種選擇時，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.
間接一般疑問句中，“if” 和 “whether” 都適用，同樣表示 “是否” 的意思。“Whether” 更正式一點。
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?