1.在引导名词性从句（多是主语从句）时，如果when和where连在一起用单数，如果分开用复数，例如：
When and where they had the party was unknown。
When they had the party and where they had it were unknown。
他们何时何地聚会的还不知道。
When and where to build the new factory is not decided yet.何时何地建新工厂还没有确定。（全国高考真题）
When and where to go for the on –salary holiday has not been decided.
何时何地去带薪假期还没有决定。（上海高考真题）
It is not known when and where exactly the tradition of sending birthday cards began ．
现在还不知道送生日卡的传统到底是从何时何地开始的。中考真题- 2019 吉林
When and where this took place is still unknown.
这件事何时何地发生的仍然是个未知数。
When and where to find him is unknown to us yet.
何时何地找到他对于我们还是个未知数。
类似句型：
What he has done and why he did it are not quite clear.
他作了什么为什么这么做还是很不明朗。
2.如果引导简单句的问句，就以其后的主语的单复数决定谓语动词用单数还是复数:
When and where does he join the Young Pioneers?
他是何时何地加入少先队的？
此句，主语是he，不是when and where。when and where是副词，分别作时间和地点状语。由于he是单数，所以用助动词does。
When and where were they playing games?
他们是何时何地玩游戏的？
they是主语，所以用were。
When and where shall we have the food festival?
我们什么时候在哪儿举办食物节?
When and where was the first stamp made?
第一张邮票是什么时候在哪里发行的？
Can you tell me when and where the conference will be held?
你能告诉我会议召开的时间和地点吗？