"which"作定语修饰整个句子时， 需要用逗号隔开。具体来说，当"which"引导的是非限制性定语从句时，它前面通常会有逗号与主句隔开。这是因为非限制性定语从句提供的是关于主句的额外信息，不是句子的必要部分。例如：
1. My sister, who lives in Paris, is coming to visit me next week.
2. The new car, which was just delivered, is very beautiful.
3. I have lost my ball, which I like very much.
在这些例子中，"which"引导的从句都修饰了整个句子，并且前面都有逗号与主句隔开。
相反，当"which"引导的是限制性定语从句时，它通常不需要用逗号与主句隔开，因为这种从句是句子的必要部分，对先行词进行具体的限定和描述。例如：
1. The book which I borrowed from the library is very interesting.
2. She showed me the photo which she had taken on her trip. [未提供具体例子，但逻辑上应该是这样]
总结来说，"which"作定语修饰整个句子需要用逗号隔开的情况是 非限制性定语从句，而 限制性定语从句则不需要。希望这能帮助你更好地理解"which"在定语从句中的用法。
