didn't need to do表示： 过去不必做某事, 事实上也没做。.
needn't have done表示： 过去不必做某事, 但事实上做了。
John went to the station with the car to meet Mary, so she didn't need to walk back home. 约翰开车去车站接玛丽，所以她不必步行回家了。
John went to the station with the car to meet Mary, so she needn't have walked back home. 约翰开车去车站接玛丽，所以她本不必步行回家了。 (Mary步行回家，没有遇上John的车。)
•when是必然發生（will happen）；if是可能發生（might happen） 1.I'll take care of it when I am free this afternoon.（這個下午有空，我就會處理。）
→一定有空 I'll take care of it if I am free this afternoon.（這個下午要是有空，我就會處理。）→可能有空，可能沒空
2.When I am in Taipei, I'll visit you.（我去台北的時候，會去拜訪你。）
→一定會去台北 If I am in Taipei, I'll visit you.（我去台北的話，會去拜訪你。）→還不確定去不去
3.天氣放晴的話，我們就會去展覽會場 （X）We may go to the fair, when the weather is bright and sunny.
（O）We may go to the fair, if the weather is bright and sunny. 人不能主宰天氣，放晴與否不一定會發生，所以用if強調不一定。
4.如果我老了，我會想和我的孩子一起生活 （X）If I am old, I'd love to live with my kids.
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