when

1.在定义的开首不该用when，如不该说Homesickness is when one longs for one's home。

2.疑问副词when所修饰的动词不该属于现在完成时态，如不可说：

• When has he done it?
• I wonder when he has done it.

该改作：

• When did he do it?
• I wonder when he did it.

但When has he done it?用作修辞疑问句便不错，作“他何曾做过这个？”解。

3.疑问副词when所修饰的动词可以属于将来时态。例如：When will he go? 和I wonder when he will go. 但关系副词when所修饰的动词往往不可属于将来时态，如不可说When he will go, I shall go with him。该把will go改作goes。比较下面三句：

• When he goes（不可用will go）, I shall go with him.
• I am looking forward to the day when I shall be able to go with him.
• A meeting will take place tomorrow, when I shall express my views on the subject.

第一句里的when引导一个状语从句，第二句里的when引导一个形容词从句，第三句里的when作and then解。

4.下面每组里的两句都可以说，似乎第二句比第一句普通：

• When we were about to start, it began to rain.
  We were about to start when it began to rain.
• When he had made up his mind, a thought occurred to him.
  He had made up his mind, when a thought occurred to him.
• When they were still laughing, the teacher came in.
  They were still laughing when the teacher came in.