定语从句中when可以换成:in/on/at+which.
具体用哪一个介词,这得根据when所指代的先行词来判断.
如:
The day when I met the famous pop star was the greatest day of my life.
when指代on the day,因此when=on which
Do you still remember the year when we worked together in the company?
when指代in the year,因此when=in which
I'll never forget the moment when I first met Mr Wang.
when指代at the moment,因此when=at which
用on which表具体某一天;用in which表年、月。