When he came I was singing.

他来的时候我在唱歌。

“when”当……时候“while”在……时候“once”一旦“though”虽然“if”如果……这些连词引导的副词从句如果它的主语跟主句的主语一致，那么这些从句可以变成分词短语。

When I saw，her I felt happy.

When seeing her，I felt happy.

当我看到她的时候我感到高兴。

While I was in Beijing，I had a good time.

While in Beijing，I had a good time.

我当时在北京过得很愉快。

When I’m unhappy，I don’t feel like talking.

When unhappy，I don’t feel like talking.

当我不高兴时，我不想说话