Structure of imperative sentances

I'm taking a Linguistics class in college, and I'm having a lot of trouble with this one subject, and the Professor isn't any help.

We're talking about the hypotheses about the structure of imperative sentances- with

VP hypothesis- imperative sentaces arent sentances, but VPs lacking a subject

Sentential hypothesis- imperative sentances contain a phonologically null subject that is syntactically second person

I don't understand how you would go about testing these hypotheses, which I probably will have to do on a quiz.

Any help would be great.

The evidence that imperatives are not VP's lacking a subject (but contain a null subject) comes from imperatives with reflexive pronouns.  Here's how the argument goes.

yourself is only used in a clause whose subject is you:

You are washing yourself.
You are keeping this for yourself.
*He is washing yourself.

So you must be the subject of these imperatives

Wash yourself.
Keep this for yourself.

even though it's not said.

CJ