Invert The Subject And Verb With Question Words

There is some confusion about when to invert the subject and verb after question words such as what, when, where, why, and how.
These words can have two very different functions in a sentence.
First, they can introduce a question, and in this case the subject and verb that follow are inverted.

What is the homework?
When can I leave?
Where are you going?

Also, these words can join together two clauses, and in this case the subject and verb that follow are not inverted.

I do not know what the homework is.
When I can leave, I will take the first train.
Do you know where you are going?

In each of these examples there are two clauses joined by a question word.
Notice that the subjects and verbs that follow the question words what, when, and where are not inverted in this case.

The following example shows how this sentence pattern could be tested in structure questions on the TOEFL test:

Example from the Paper and Computer TOEFL® Tests
The lawyer asked the client why ___ it.
(A) did he do
(B) did he
(C) he did
(D) did

In this example, the question word why is used to connect the two clauses,
so a subject and verb are needed after this connector; this is not a question, so the subject and verb should not be inverted. The best answer is therefore answer (C).