i often think of the day----you were with us

you were with us 修饰 the day .the day 是时间，故应用when 引导从句。

因为day是时间呀，而且有持续性，于是用when连接。

这句话的意思是我常常想起你和我们在一起的日子，这应该是一个时间状语从句，用when引导

the day 是you were with us 的时间状语。