"When are they leaving?" 是询问"他们什么时候出发"的常见英语表达方式。
来源：这种表达方式可以归结为主语（they）和谓语（are leaving）。其中，"when"是一个疑问词，表示“什么时候”；"are"是动词，表示"是"的意思； "leaving"是动名词，表示"出发"的意思。
例句：
"When are they leaving for the trip?"（他们什么时候出发旅行？）
"I was wondering when they were going to leave." ( 我在想他们什么时候出发)
"I need to know when they are leaving so I can plan accordingly." (我需要知道他们什么时候出发，这样我才能做好计划)