急：为什么What do you think is the most helpful invention?和what do you think I can do?语序不同

What do you think is the most helpful invention?这里是插入语,插入语不影响句子的成分结构.
what do you think I can do?这里特殊疑问词和what和 do you think组成混合疑问句,不是插入语,所以句子要用正常语序.那做题时又怎么区分do you think到底是插入语还是组成混合疑问句呢，依靠什么作为选择依据呢既然做插入语不会影响句子那就 把do you think 去掉 如果句子语序正常不受影响那就是插入语，如果受影响，则是疑问这样的话，如果把what do you think I can do?改为what do you think can I do? 句子也应该是对的把，改后的句子中 do you think 就是插入语了。What do you think is the most helpful invention？ 也可以改为混合疑问句What do you think the most helpful invention is？ 可以吗？为什么不可以？