第一句中 正常语序为：That is what book on the table.可以看出,that是指示代词做主语,不能改.
第二句中 which the teacher explained yesterday 可以看作表语从句.Is this problem （ ）the teacher explained yesterday?A thatB the oneC which那为什么不选which正确的答案应是B， the one。这样，the one就作了表语，后面的the teacher explained yesterday作定语从句，修饰the one，关系代词作explain的宾语省略了。在这里不能用which. which一般是对定语提问。如：The book on the desk is mine. 如果对句子on the desk提问的话，要用which, 即：Which book is yours？而在（ ）the teacher explained 句中， 括号内是做explained的宾语，所以不能用which, 但可以用what. 我是语法通霸一书的编者，如有疑问，可以继续问我。