Do you know how thunderstorms are formed?
Do you know how a thunderstorm is formed?
Can you tell me who that girl is?
To form an object (e.g., that girl is) take the statement and move who to the front, like this,
Statement: That girl is who?
Object: . . . who that girl is?
Example: Can you tell me who that girl is?
That girl is the subject, is is the verb and who is the object, or in this case, since is is a form of the verb BE, who functions as a predicate nominal, technically speaking.
She is who?
I wonder who she is.
The next example does not require WH-movement. 'Who' is the subject,
John is reading this.
Who is reading this?
I wonder who is reading this? (grammatical)
The example below requires WH-movement because what functions as an object in that sentence. When the WH-word functions as a subject movement isn't required:
She saw what?
I wonder what she saw?
I wonder what she did see? (did is added for emphasis)
She sees what?
I wonder what she sees?
I wonder what she does see? (does is added for emphasis)
Thunderstorms are made how?
Can you tell me how thunderstorms are made?
Hope that helps out some.