when在英語裏有三種詞性：
特殊疑問詞，“什麽時候”，後面跟壹般疑問句語序。如：
When were you born?
When will the class begin?
When do you start work and finish work?
從屬連接詞，“當什麽時候”，引導時間狀語從句，後接陳述句的語序，如：
He always helps me when I am in trouble.
When my dad came back, I was watching TV.
縮合連接副詞，引導名詞性從句。
This was when I was born.
Do you know when the class will begin?
I don't know when they will achieve the goal.
此外，疑問副詞when後面加動詞不定式to do（註意，when後面沒有接動名詞doing的用法），可以作句子的主語，表語、賓語等，如：
I don't know when to use the sentence pattern.
When to go is a problem.
The problem is when to begin the class.