VOA一分钟英语：Not need to 和 needn't 区别是什么？

动词 “need” 的意思是 “需要，必须”。它的否定形式有两种：“not need to” 和 “needn’t”。它们有区别吗？现在时态和过去时态下的这两个否定形式在用法上一样吗？
 
Hi, everybody. I’m Sam, and in this lesson I’m going to talk about ‘need’. Do you know the difference between needn’t and not need to?
Look at these two sentences in the present – is the meaning the same or different?
You don’t need to worry about the exam, you’ve studied lots.
You needn’t worry about the exam, you’ve studied lots.
They’re the same, right? Maybe needn’t is a little bit more formal, but the meaning is that there is no necessity for you to worry.
Now look at these two sentences in the past:
He didn’t need to worry about the exam. He had already passed the course.
He needn’t have worried about the exam. It was really easy.
These are different! In the first sentence, he had already passed the course, so he knew he didn’t need to worry. In the second sentence, he didn’t know the exam was going to be easy, so he was worried but he needn’t have been.
 
用法总结
1 在现在时态下，“(do) not need to” 和 “needn’t” 表达的意思通常是一样的，即 “不需要，用不着”。“Needn’t” 比 “not need to” 稍微更正式一点。
You don’t need to worry about the exam, you’ve studied lots. 
You needn’t worry about the exam, you’ve studied lots.
I don’t need to go to work tomorrow as it’s a holiday.
I needn’t go to work tomorrow as it’s a holiday.
 
2 在过去时态下，“(did) not need to” 常用来表达我们之前没有做某事，因为 “不需要”。
He didn’t need to worry about the exam. He had already passed the course.
I didn’t need to go to work today, so I stayed in bed.
 
3 在过去时态下，“needn’t + have + 动词过去分词” 常用来表达我们之前做了某事，但其实 “本不必” 做。
He needn’t have worried about the exam. It was really easy.
I needn’t have gone to work today. I didn’t realise it was a holiday. What a mistake!