求教：一条高一定语从句的问题
Is this pen_________you lost yesterday?
补充成定语从句的话 是不是填the one which
但如果直接what可不可以呢
Suzhou,where I spent my childhood,I have never been back to.
这句话有没有问题？要表达这个意思，应该怎么说？
The reason for which he was late for school is that he got up late this morning.
请问这个句子要怎么理解？这个which代什么呢？