牢记一条.按主谓宾的顺序排列的是陈述语序.
which do you like best 陈述是 you like which best .其中which做like宾语.用do做助动词,放在like前面提问.
what were they doing 陈述是what they were doing或者they were doing what. were doing 这种有be动词形式的疑问句就是把be提前.
单独看which is your favorite book这个句子是问句啊.
换成what也还是问句.
但是如果放在句子中做从句的话.which此时就做了代词,成为了后面从句的主语.句子主语在谓语is之前,那当然就是陈述语序啊.eg：I don't know what is your favorite book.我不知道你最喜欢的是什么书.