Verb Pairing - How to explain?

I am applying to do a TESOL course. The test paper is asking me to correct sentences and state what rules are broken. I am stuck on explaining two - could anybody help me please?

a. Do you can understand?

b. He asked what do I earn (keep in indirect form)

The answers I have given are; Do you understand, and He asked what I earn. I see a double use of verbs in the sentence but am struggling to give a better explaination.