定語從句when的用法：
關係副詞when主要用於修飾表時間的名詞，同時它在定語從句中用作時間狀語。如：
There comes a time when you have to make a choice. 你必須作出抉擇的時候到了。
Gone are the days when they could do what they liked. 他們為所欲為的日子一去不復返了。
We’ll put off the picnic until next week, when the weather may be better. 我們將把野餐推遲到下星期，那時天氣可能會好一點。
注意不要一見到先行詞為時間名詞，就以為一定要用關係副詞when來引導定語從句，同時還要看它在定語從句中充當什麼成分——如果在定語從句中用作時間狀語，就用when;如果在定語從句中不是用作時間狀語，而是用作主語或賓語，那就不能用when，而要用that, which等。如：
Don’t forget the time (that, which) I’ve told you. 不要忘記我告訴你的時間。
關係代詞that / which在定語從句中用作動詞told的賓語，正因為是用作賓語，所以也可以省略。
【拓展延伸】
定語從句用法歸納
定語從句由關係代詞who、whom、whose、which、what、as和關係副詞where、when、why等引導，但須記住：
不能引導定語從句.
2.關係詞的分析須考慮它在定語從句中的成分。
一.指人的.關係代詞有who、whose、whom、that. 試分析：
The old man who/whom /that we visited yesterday is a famous artist.
Miss Wang is taking care of the child whose parents have gone to Beijing .
The man with whom my father shook hands just now is our headmaster. (=The man who/whom that my father shook hands with just now is our headmaster.)注：
A. 指人時有時只用who不宜用that。
1.先行詞為one、ones或anyone
(1)The comrade I want to learn from is the one who studies hard and works hard.
(2)Anyone who breaks the law should be punished.
2.先行詞為these時
These who are going to Beijing are the best students of our school.
3.在there be 開頭的句子中
There is a student who wants to see you.
4.一個句子中帶有兩個定語從句，其中一個定語從句的關係代詞是that，另一個宜用who，以免重複。
The student that won the first prize is the monitor who works hards.
5.在非限制性定語從句中
A。I met a friend of mine in the street, who had just come from Japan.
B.主句以who開頭的句子中,只用that,不用who。
