when引导的时间状语可以是现在时,主句用一般将来时(情态动词,祈使句）,如：
When he comes,I will tell you.
when引导的时间状语用过去时,主句用过去进行时,如：
I was having breakfast when he arrived.
when引导的时间状语用过去时,主句用过去完成时,如：
When we got to the station,the train had already left.
when引导的时间状语用过去进行时,主句用过去时,如：
He arrived when I was taking breakfast.