不太好说清楚,我可以举个例子.
This is a time when A and B were in love.A和B在这个时期是相爱的.
There is a time when A and B were in love.A和B曾经有过相爱的时光.
两者强调的重点不一样.跟的从句一般是过去时吧.当然时态是很灵活的,要看语境了.